Thursday’s testimony included two medical witnesses who helped underscore prosecution claims that Mr. Floyd died from being held down by Mr. Chauvin, not drug use, as the defense argues.

George Floyd showed signs of a brain injury 4 minutes before Derek Chauvin lifted his knee, a doctor testifies.

The court is taking its afternoon break, but Derek Chauvin’s lawyer has raised a concern that prosecutors gave him an updated version of documents that was not clear to him. The judge said he did not believe there was anything nefarious at play.

The upshot of the state’s examination of Dr. Daniel Isenschmid was to show that George Floyd’s levels of fentanyl and meth were much lower than many DUI cases he had worked on in which the suspect lived.

Derek Chauvin’s lawyer pushed back against a lung doctor’s testimony on Thursday that Mr. Chauvin’s knee had been a cause of George Floyd’s death, suggesting during his cross-examination that Mr. Floyd’s death may have been an overdose.

The argument from Mr. Chauvin’s lawyer, Eric J. Nelson, came after more than two hours of testimony from Dr. Martin J. Tobin, a pulmonologist and critical care doctor, in which Dr. Tobin said Mr. Chauvin had been a significant cause in Mr. Floyd’s death. Dr. Tobin said that Mr. Floyd’s breathing rate, in the moments before he died, indicated that the fentanyl in his system was not having any effect on his breathing.

Fentanyl and methamphetamine were found in Mr. Floyd’s system, and the possibility that they caused his death is a crucial argument for Mr. Chauvin’s defense as the trial enters a phase in which medical testimony is front and center.

Mr. Nelson suggested on Thursday that Mr. Floyd could have died of a fentanyl overdose if he had taken the drug in the moments before police officers pushed him to the ground. Dr. Tobin said that, unlike someone who died of a fentanyl overdose, Mr. Floyd had never gone into a coma. Mr. Nelson also sought to portray the medical care that Mr. Floyd received from paramedics as lacking; it took them nine minutes to insert a tube down Mr. Floyd’s throat after they arrived on scene, he said, but the nearest hospital was a five-minute drive away.

Mr. Nelson also noted that even doctors sometimes have trouble understanding that a patient who can talk is having trouble breathing. Mr. Floyd told police officers that he could not breathe, and in response, one officer told him that it takes oxygen to talk, implying that he must be able to breathe, something medical experts have said is false or misleading.

Dr. Tobin, who works in Chicago at Edward Hines Jr. Veterans Affairs Hospital and at Loyola University’s medical school, was called as an expert by prosecutors. He had testified on Thursday morning that Mr. Floyd’s death was a result of a lack of oxygen and that Mr. Floyd had showed signs of a brain injury about four minutes before Mr. Chauvin lifted his knee from his neck. He said that in addition to the pressure from Mr. Chauvin’s knees, the oxygen depletion was also caused by Mr. Floyd’s hands being cuffed behind him and his body being prone against the pavement.

In an effort to bolster the testimony last week from George Floyd’s girlfriend that he had a high tolerance for fentanyl, the prosecution asks Dr. Daniel Isenschmid to describe how a high tolerance impacts the effects of the drug on someone. This is a continued attack on the defense’s drug overdose theory.

The prosecution is continuing to attack the defense assertion that George Floyd died of a drug overdose. Dr. Daniel Isenschmid testified that the presence of a certain type of fentanyl that Floyd had in his system suggests that there was not an overdose because that type is not typically found in fatal overdose victims.

Dr. Daniel Isenschmid testified that the amount of methamphetamine found in George Floyd was consistent with a dose of prescription meth. He called it a low amount. The defense has argued that Floyd died in part of a drug overdose.

Now testifying is Dr. Daniel Isenschmid, a forensic toxicologist. He is a former medical examiner, and we’ll likely hear more from him about the causes of George Floyd’s death.

Daniel Isenschmid, the forensic toxicologist who performed the toxicology tests for an autopsy of George Floyd, was called to the stand on Thursday.

Dr. Isenschmid works as a forensic toxicologist at NMS Labs, a toxicology and forensic sciences laboratory that provides analysis services to law enforcement, medical examiners, coroners, doctors, lawyers and others.

Dr. Isenschmid is also a fellow at the Center for Forensic Science Research and Education and previously worked with the Office of the Wayne County Medical Examiner in Detroit, according to a biography on the Center’s website. He is certified by the American Board of Forensic Toxicology.

The Hennepin County Medical Examiner sent NMS Labs samples from Mr. Floyd, including hospital blood and urine collected at autopsy, he said.

The lab found 11 nanograms per millileter of fentanyl and 19 nanograms per millileter of methamphetamine in the blood that was tested, Dr. Isenschmid said. He said that the level of methamphetamine found was low.

The report also found 86 nanograms per millileter of morphine in the urine that was tested, but not the blood. This could indicate a prior use, as it can remain present in urine longer than blood, he said.

In the widely viewed bystander video of George Floyd’s arrest, it seemed plain that Derek Chauvin had positioned his left knee on Mr. Floyd’s neck.

But Mr. Chauvin’s defense has repeatedly suggested, and tried to get witnesses to agree, that his knee was actually on Mr. Floyd’s shoulder.

The most obvious reason for this is that the medical examiner, Dr. Andrew Baker, attributed Mr. Floyd’s death in part to neck compression.

But several experts said it did not matter: The knee could be lethal in either position, neck or shoulder.

“In my opinion, does it make a difference?” asked Dr. Alon Steinberg, a cardiologist who has published a review of the scientific literature on prone restraint deaths. “No.”

Dr. Steinberg said that multiple factors including the restriction of breathing and of blood flow contribute to death during prone restraint.

Dr. Judy Melinek, a forensic pathologist who has written extensively about the case, said, “Even in the absence of pressing on the neck, you’re putting strain on the cardiovascular system, especially if somebody is fighting or struggling against the restraint.”

The prosecution on Thursday appeared to be pursuing a similar argument. The first witness was Dr. Martin Tobin, a pulmonologist who specializes in the mechanics and physics of breathing.

He said that Mr. Floyd died from a “low level of oxygen” that led to brain damage and an arrhythmia that “caused his heart to stop.” Dr. Tobin said terms such as hypoxia or the prosecution’s preferred word, asphyxia, are “really other words for a phenomenon that is a low level of oxygen.”

Mr. Floyd’s breathing was restricted, Dr. Tobin explained, by the way the officers pushed in his cuffed hands, squeezing his rib cage against the hard pavement, and by Mr. Chauvin’s placement of his left knee on Mr. Floyd’s neck and right knee on his back.

Mr. Chauvin’s left knee was on Mr. Floyd’s neck “more than 90 percent of the time,” Dr. Tobin said, and his right knee was on his back at least 57 percent of the time. Dr. Tobin said that for the rest of the time, the available video did not provide a good view of the location of Mr. Chauvin’s right knee.

Dr. Martin J. Tobin ends his testimony with one last explanation of why he believes fentanyl was not responsible for George Floyd’s death. People who die of a fentanyl overdose go into a coma, he said, but Floyd never did.

On re-direct of Dr. Martin J. Tobin, the prosecution is establishing that not everything can be seen in an autopsy. That’s a crucial foundation to lay because prosecutors are departing from the medical examiner’s autopsy results.

Dr. Martin J. Tobin also addressed an issue that the defense has brought up — that George Floyd didn’t have bruising on his neck. That doesn’t mean anything, Dr. Tobin said. “Whenever I go to church I sit on a hard bench,” he said. “I don’t get bruising of my buttocks when I leave, so I wouldn’t expect anything in terms of that.”

In the morning, Dr. Martin J. Tobin said fentanyl had no effect on George Floyd’s breathing. To try to challenge that assertion, Eric Nelson, Derek Chauvin’s lawyer, is pointing out the difference in quality between pharmaceutical-grade fentanyl and fentanyl bought on the street, the type presumably Floyd ingested. Another small way Nelson is trying to raise reasonable doubt.

This is the challenge for a lawyer trying to poke holes in the testimony of a renowned scientific expert: The expert knows more than the lawyer, so he’s able to swat away some of the lawyer’s suggestions because they’re not specific enough.

Dr. Martin J. Tobin has corrected Eric Nelson, the lawyer for Derek Chauvin, a number of times. One key issue the defense has put forward is that when George Floyd said he couldn’t breathe, the very fact he was speaking meant he could breathe. Tobin says that is irrelevant because a few seconds after someone says they can’t breathe, they can be dead.

The hope of Derek Chauvin’s lawyer here is to do whatever he can to obfuscate Dr. Martin J. Tobin’s findings and in essence confuse the jury – make them think that what Dr. Tobin said in the morning with so much certitude might be more complicated. One line of attack: pointing to the autopsy’s findings that there was no bruising on George Floyd’s neck.

Eric Nelson, Derek Chauvin’s lawyer, starts by pointing out that Dr. Martin J. Tobin didn’t accept a fee to testify for the state. The lack of payment, experts say, can cut both ways. On one hand, not accepting payment can bolster the witness’ testimony because it shows he wasn’t paid for it. On the other hand, as Nelson hopes to imply, it could suggest the witness has such a strong opinion – therefore bias – about the case that he is willingly to do it for free.

We are back after the lunch break, with Derek Chauvin’s lawyer set to cross examine Dr. Martin J. Tobin.

In 2016, Greg Traylor was working as a janitor in the emergency department where Philando Castile was brought after being fatally shot by a police officer in a Minneapolis-St. Paul suburb. Now, as a security guard for Hennepin County libraries, Mr. Traylor regularly struggles with his and others’ emotions about last year’s police killing of George Floyd.

“George Floyd’s death hurt and angered me,” said Mr. Traylor, a 30-year-old Black man. “I thought, OK, I know how police try to get their officers off in these situations, but this time there should be no excuses.”

Mr. Traylor said he had moved to the Twin Cities in part because Chicago, his hometown, was too violent. The death of Mr. Castile, who was killed during a traffic stop, opened his eyes.

“Minnesota’s got a whole lot of good things going on, but no place is perfect,” he said. “I’m afraid cops and Blacks will always be in conflict in this country.”

While marching in protests after Mr. Floyd’s death, Mr. Traylor carried a sign that said, “Four Cops Arrested — I’ll Be Happy With Four Cops Convicted.” He was overwhelmed by the outpouring of support, including from many white people.

But although he sometimes discusses Derek Chauvin’s trial with colleagues, Mr. Traylor said he had to limit the time he spent watching the coverage. “I can’t go to work with that heaviness and pain on my mind,” he said.

For his security job, Mr. Traylor carries mace and a baton. He has been trained in the reasonable use of force, he said, and thought the evidence was overwhelming that Mr. Chauvin had committed a crime.

“We don’t need this 20-day trial; it’s on camera,” he said of Mr. Floyd’s death. “I think it’s going to work out in everybody’s favor.

The court is taking a break for lunch until 1:30 Central Time after more than two hours of testimony from a lung doctor. After the break, Derek Chauvin’s lawyer will have a chance to cross-examine the doctor.

A veteran lung doctor testified on Thursday that George Floyd’s death was caused in part by Derek Chauvin’s knees pressing against his neck and back, making it impossible for him to breathe, and that Mr. Floyd showed signs of a brain injury about four minutes before Mr. Chauvin lifted his knee from his neck.

Dr. Martin J. Tobin, a pulmonologist and critical care doctor in Chicago, said in court that the combination of Mr. Chauvin’s pressure, the handcuffs pulling Mr. Floyd’s hands behind his back and Mr. Floyd’s body being pressed against the street had caused him to die “from a low level of oxygen.”

The testimony from Dr. Tobin on the ninth day of the trial of Mr. Chauvin, the former Minneapolis police officer who has been charged with murdering Mr. Floyd, signaled a shift into a new phase in which medical testimony will be crucial. Mr. Chauvin’s lawyer has suggested that Mr. Floyd died from the fentanyl and methamphetamine that were found in his system, but prosecutors argue that Mr. Chauvin had killed him by kneeling on him for more than nine minutes and cutting off his air supply.

Dr. Tobin was adamant that Mr. Chauvin’s actions had caused Mr. Floyd’s death on May 25. He pinpointed the exact moment, in the graphic bystander video of the arrest, in which he said Mr. Floyd had died, noting that his eyes had opened wide and then closed again. At that point, Mr. Chauvin’s knee remained on his neck.

“You can see his eyes — he’s conscious — and then you see that he isn’t,” Dr. Tobin said. “That’s the moment the life goes out of his body.”

The doctor also rejected the defense’s arguments about drugs, saying that videos show Mr. Floyd breathing at a normal rate before he went unconscious, meaning any fentanyl in his system was “not having an effect” on his breathing.

“A healthy person subjected to what Mr. Floyd was subjected to would have died,” Dr. Tobin said.

Eric J. Nelson, the lawyer for Mr. Chauvin, pushed back on Dr. Tobin’s testimony during his questioning and suggested that Mr. Floyd could have died of an overdose. Dr. Tobin agreed that fentanyl could have hindered Mr. Floyd’s breathing if he had taken the drug in the minutes before police officers pushed him to the ground, but the doctor said Mr. Floyd had never gone into a coma, something he said would happen before a person dies of a fentanyl overdose. Mr. Nelson also sought to portray the medical care that Mr. Floyd received from paramedics as lacking, saying they had not inserted a tube down Mr. Floyd’s throat until nine minutes after they arrived.

In his testimony, Dr. Tobin said he had determined that Mr. Chauvin had pressed his left knee on Mr. Floyd’s neck for more than 90 percent of the time that Mr. Floyd was on the ground, and that he had kept his right knee on Mr. Floyd’s back for the majority of the time as well. That pressure, combined with having his hands cuffed behind his back and pushed into the street facedown, had cut off oxygen and caused his heart to stop, Dr. Tobin said.

Mr. Floyd was so desperate for air at one point that he tried to lift himself off the ground by pushing his right knuckle against a police car’s tire, the doctor said.

Dr. Tobin, who works in pulmonology and critical care at Edward Hines Jr. Veterans Affairs Hospital and at Loyola University’s medical school, spoke for more than two hours to jurors. He said he had asked not to be paid when prosecutors asked if he would testify in the case. At several points, he encouraged jurors to feel parts of their own necks to demonstrate what he was saying; most of them followed along.

Dr. Tobin said the factors that cut off Mr. Floyd’s air supply had led to what could be called “asphyxia,” which he said was just another word for oxygen deprivation; a prosecutor told jurors when the trial began that Mr. Floyd had died of asphyxia.

At one point in a video of Mr. Floyd’s arrest, Dr. Tobin noted, Mr. Chauvin’s left foot appeared to rise off the ground, which the doctor said meant that half of Mr. Chauvin’s body weight was pressing on Mr. Floyd’s neck.

After about 4 minutes and 51 seconds, Dr. Tobin said, Mr. Floyd stopped speaking or groaning. After just over 5 minutes, Mr. Floyd appeared to straighten out his legs, which Dr. Tobin said was a signal that Mr. Floyd was having a type of seizure because he had suffered a brain injury from the oxygen deprivation. He said Mr. Chauvin’s knee had stayed on Mr. Floyd’s neck for 3 minutes and 27 seconds after he took his last breath.

The doctor said the handcuffs were also an “extremely important” factor in Mr. Floyd’s death because Mr. Chauvin and another officer had pushed his hands upward and against his back, pressing his chest against the hard street.

“When you’re turned prone and with the knee on the back, now the work that Mr. Floyd has to perform becomes huge,” Dr. Tobin said, adding: “He has to try to lift up the officer’s knee with each breath.”

Dr. Martin J. Tobin is turning out to be kind of a star because he’s not just opining, he’s done calculations to support all his points — from air intake to actually counting George Floyd’s breaths.

A lot of big words here, but an important point by Dr. Martin J. Tobin about George Floyd’s breathing. He essentially said that Floyd’s breathing did not slow down enough to say that fentanyl was a factor in his not getting enough oxygen. In other words, he was breathing at a normal rate before he became unconscious. The defense has argued that fentanyl was a cause of Floyd’s inability to breathe.

Dr. Martin J. Tobin also says the increased level of carbon dioxide in George Floyd’s body was not because of fentanyl, but because he wasn’t given any ventilation for nearly 10 minutes.

One thing that Dr. Martin J. Tobin is doing quite effectively is clarifying that multiple terms — hypoxia, asphyxia, anoxia — all mean essentially the same thing. He’s saying the jury doesn’t need to worry about the medical terminology because the words all refer to a “drastically low level of oxygen.”

We think of asphyxia as a total cutoff of oxygen, like with strangulation. The prosecution is trying to get the jury to understand that reduced oxygen flow can also be lethal.

Tobin says that the fact that Floyd was speaking creates “a huge false sense of security,” that just because someone is speaking (expelling air) does not mean he is not about to die.

Addressing the fact that George Floyd was found to have a type of tumor called a paraganglioma, Dr. Martin J. Tobin said he did not consider it relevant to Mr. Floyd’s death, adding that the medical literature cites only six “sudden deaths” caused by paraganglioma.

The defense has argued that George Floyd died because of his poor health and because he had drugs in his system. Dr. Martin J. Tobin said even a healthy person would have died under similar circumstances. “A healthy person subjected to what Mr. Floyd was subjected to would have died as a result of what he was subjected to,” he said.

Dr. Martin J. Tobin said that by 8:25 p.m., George Floyd had no more oxygen in his body. Derek Chauvin’s knee remained on Floyd’s neck for another 3 minutes 2 seconds after that, he testified. That sounds pretty gruesome.

One really effective thing that the prosecution is accomplishing with Dr. Martin J. Tobin: They’re making it irrelevant whether Derek Chauvin’s knee was actually on George Floyd’s neck versus his back or shoulders. It’s all bad for his breathing is the main takeaway. That might be a reason why prosecutors are going with asphyxia (oxygen deficiency) as the cause of death, rather than what the medical examiner determined, “cardiopulmonary arrest,” which means the heart stops.

Dr. Martin J. Tobin is now getting into some complicated terminology about oxygen reserves, but it all points to a general theme: George Floyd being in a prone position, handcuffed with a knee in his neck, led to a reduction in his oxygen reserves.

“There’s a huge increase in the work that Mr. Floyd is performing just to cope with what was happening below the neck,” Tobin said.

Notably, no talk yet of drugs during the state’s examination of Dr. Martin J. Tobin. I wonder if prosecutors will bring it up when testimony resumes, in an effort to get ahead of what is likely to be Derek Chauvin’s lawyer’s line of questioning when he cross-examines Tobin.

Dr. Martin J. Tobin has done a moment-by-moment analysis of the officers’ positions on George Floyd’s body. The doctor said he focused on the first five minutes and three seconds because that is the point when he saw evidence of brain injury. The officers held Floyd down for nine minutes, 29 seconds.

Despite how complicated Dr. Martin J. Tobin’s testimony has been, one of the reporters in the courtroom today says that most jurors were watching intently and taking notes. Most followed along with Tobin and touched their own necks as he gave his testimony and described how George Floyd died.

This is an analysis of inches. Dr. Martin J. Tobin points to a photo in which Derek Chauvin’s toe is slightly off the pavement as he kneels on George Floyd’s neck. “This means that all of his body weight is being directed down at Mr. Floyd’s neck,” Tobin said.

This equates to half of Chauvin’s body weight — or 91.5 pounds — coming down on Floyd’s neck, the doctor says.

Dr. Martin J. Tobin, a renowned expert on breathing, was a key witness for the prosecution on Thursday, agreeing to testify without pay to offer his opinion on what caused George Floyd’s death:

Mr. Floyd died from a low level of oxygen and this caused damage to his brain that we see and it also caused a P.E.A. arrhythmia that caused his heart to stop.

The testimony was an attempt to discredit defense arguments that Mr. Floyd’s drug use contributed to his death. Using video from the arrest, Dr. Tobin described how Derek Chauvin’s knees on Mr. Floyd’s neck and side and his hold on Mr. Floyd’s arms prevented Mr. Floyd from being able to breathe.

He’s jammed down against the street, and so the street is playing a major role in preventing him from expanding his chest.

He also showed two photos of Mr. Floyd’s finger and knuckles digging into the street and the police car’s tire as the police held him down:

To most people this doesn’t look terribly significant. But to a physiologist this is extraordinarily significant because this tells you that he has used up his resources and he is now literally trying to breathe with his fingers and knuckles.

Dr. Tobin said people who can’t breathe normally will use other ways to try to bring more air into their body:

So he’s using his fingers and his knuckles against the street to try to crank up the right side of his chest. This is his only way to try to and get air into the right lung.

The judge notes, after a private conversation with lawyers for both sides, that the jurors do not have to touch their necks, as the doctor is doing in order to describe what happened to George Floyd. It seems likely that Derek Chauvin’s lawyer requested that instruction.

I wonder if Derek Chauvin’s lawyer senses that doing what the doctor says could make the jurors sympathize more with what George Floyd went through.

Dr. Martin J. Tobin has explained that George Floyd’s vocalizations were really important. From the opening statement by the prosecution: “You will hear his final words when he says ‘I can’t breathe.’ Before that time, you’ll hear his voice get heavier. You will hear his words further apart. You will see that his respiration gets shallower and shallower and finally stops when he speaks his last words, “I can’t breathe.”

Dr. Martin J. Tobin is showing us how to examine our necks with his hands. Don’t know about you all, but it’s kind of hard for me to touch my neck. I get kind of freaked out. I wonder if jurors are doing it and if that helps them better understand what George Floyd went through.

As the trial nears a phase where George Floyd’s cause of death will take center stage, we talked with several forensic pathologists uninvolved in the case to explain some of the terms used in the proceedings, how they determine the cause and manner of death and how this relates to the case. Here is what we learned.

A lot of the discussion in this case has been about Derek Chauvin’s knee on George Floyd’s neck. But Dr. Martin J. Tobin seems to indicate that a knee on the back or the side has a similar effect in terms of limiting Floyd’s ability to breathe.

Dr. Tobin gives a direct opinion on what caused George Floyd to die: “Mr. Floyd died from a low level of oxygen and this caused damage to his brain.” He also said that that low level of oxygen “caused his heart to stop.” This is the first direct testimony we’ve had that Floyd died from Derek Chauvin’s knee on his neck, back and side.

You may be hearing the phrase “P.E.A.” arrythmia — pulseless electrical activity. It’s a type of heart problem that is often cited when people die while being restrained in a prone position. It’s different from other types of arrhythmia, such as those that result from cardiovascular disease, and you can’t use a defibrillator to restart the heart in a P.E.A. case.

It’s really interesting that Dr. Tobin just said most of his testimony has been in medical malpractice cases and he’s never testified in a criminal case before — he cannot be viewed as biased for or against law enforcement.

After leading Dr. Martin J. Tobin through his credentials, the state’s questioning gets to the central issue of the case: “I’m primarily interested in breathing,” he says.

“So let’s talk about your opinions with respect to this case, have you formed an opinion to a reasonable degree of medical certainty on the cause of Mr. Floyd’s death?” “Yes, I have.” “Would you please tell the jury what that opinion, or opinions, are?” “Yes, Mr. Floyd died from a low level of oxygen, and this caused damage to his brain that we see. And it also caused a P.E.A. arrhythmia that caused his heart to stop. The cause of the low level of oxygen was shallow breathing, small breaths, small tidal volumes, shallow breaths that weren’t able to carry the air through his lungs, down to the essential areas of the lungs that get oxygen into the blood and get rid of the carbon dioxide — that’s the alveoli at the bottom of the lung. The main forces that are going to lead to the shallow breath are going to be that he’s turned prone on the street, that he has the handcuffs in place combined with the street, and then that he has a knee on his neck, and then that he has a knee on his back and on his side. All of these four forces are ultimately going to result in the low tidal volume, which gives you the shallow breaths that we saw here. And so the air will not be able to reach those air sacs — we just saw in the video where the oxygen is exchanged and the carbon dioxide is removed. What you’re seeing is the orientation of Officer Chauvin, his body build is quite erect here. But in particular, what you’re seeing is that the toe of his boot is no longer touching the ground. This means that all of his body weight is being directed down at Mr. Floyd’s neck.”

Dr. Martin J. Tobin, a pulmonologist and critical care physician from the Chicago area, was the first witness prosecutors called to the stand in the Derek Chauvin trial on Thursday.

When prosecutors asked if he had formed a medical opinion on what had caused George Floyd’s death, Dr. Tobin said, “Mr. Floyd died from a low level of oxygen, and this caused damage to his brain that we see, and it also caused a P.E.A. arrhythmia because his heart stopped,” referring to pulseless electrical activity, or cardiac arrest.

The low level of oxygen was caused by “shallow breathing,” he said. Mr. Floyd’s prone position and his being handcuffed, and Mr. Chauvin’s knee on his neck and back, he said, contributed to the shallow breathing.

Dr. Tobin added that the position in which Mr. Floyd was handcuffed, with the force of the officers compounding with the asphalt street, ultimately prevented him from being able to breathe fully.

“It’s how the handcuffs are being held, how they’re being pushed, where they’re being pushed that totally interfere with central features of how we breathe,” he said.

Mr. Chauvin’s knee placed on the left side of his chest would have limited the amount of air being able to enter the left lung, Dr. Tobin said, as if “a surgeon had gone in and removed the lung,” he said.

Dr. Tobin attended medical school at University College Dublin and is an expert in acute respiratory failure, mechanical ventilation and neuromuscular control of breathing.

He is certified by the American Board of Internal Medicine and the American Board of Internal Medicine Subspecialty Pulmonary Disease.

In the prosecution’s opening statement, Jerry Blackwell said of George Floyd: “You will learn he’s pancaked with a hard pavement beneath him and Mr. Chauvin on top of him. In order to breathe, you have to have room for the lungs to expand in and out. And you’ll see Mr. Floyd doing his best to kind of crank his right shoulder, having to lift up his weight and Mr. Chauvin’s weight on top of him to get a breath.”

This is likely what Dr. Tobin will go over today. He has just said that his studies of breathing are “more in the realm of math and physics” than medicine.

The reason they are going through Dr. Martin J. Tobin’s credentials so extensively is because the prosecution has to do all that it can to bolster his expertise with the jury. So we’ll be hearing these long bios from all of the expert witnesses.

Today the state’s case moves into its third phase – medical testimony concerning the cause of George Floyd’s death. It is likely to be less emotional than the bystander testimony last week, and perhaps less definitive than police witnesses who said Chauvin’s use-of-force was excessive. But it could be more important, because cause of death is the most contested issue of the trial.

In Minneapolis, educators have grappled over the last few weeks with how to talk with their students about the trial of Derek Chauvin, the former Minneapolis police officer charged with murder in George Floyd’s death.

Some teachers have shown segments of the televised trial in class and used jury selection or witness testimony as an opportunity to explore the complex issues of race, policing and the criminal justice system. Others have cautiously given students the chance to ask questions and share their opinions. And school administrators and counselors have scheduled talking circles, where children can open up about how the trial has rekindled feelings of racial trauma and fears of potential unrest stirred by the sound of helicopters flying over the city.

But the adult nature of the televised murder trial, marked by graphic videos and emotional eyewitness accounts, poses a challenge for educators, even as they say the court proceedings are too important to ignore.

“We have to engage even if we’re uncomfortable and we don’t have the answers,” said Kristi Ward, the principal for third through eighth graders at Lake Nokomis Community School in south Minneapolis.

The trial of Derek Chauvin will continue on Thursday with a renewed focus on George Floyd’s drug use, after a day of testimony that suggested Mr. Floyd had drugs with him when he was arrested in Minneapolis last May.

Mr. Floyd’s drug use has been a primary focus of the defense of Mr. Chauvin, the former police officer who is charged with murdering Mr. Floyd.

On Wednesday, witnesses described the Minnesota Bureau of Criminal Apprehension’s investigation into Mr. Floyd’s death, explaining that pills and pill fragments found at the scene contained methamphetamine and fentanyl. The defense, led by the lawyer Eric J. Nelson, has suggested that Mr. Floyd died from complications of drug use.

The jury also heard from a use-of-force expert who said Mr. Chauvin used “deadly force” even though Mr. Floyd posed no immediate threat to the officers. “He was in the prone position, he was handcuffed, he was not attempting to resist, he was not attempting to assault the officers — kick, punch, or anything of that nature,” said the expert, Sgt. Jody Stiger, who works with the Inspector General’s Office for the Los Angeles Police Department.

Sergeant Stiger said that Mr. Chauvin might have been justified in using some force, such as a Taser, earlier in the arrest, when Mr. Floyd resisted as officers tried to put him in the back of a police cruiser. But once Mr. Floyd was handcuffed and facedown on the street, the force should have stopped, the sergeant said. Mr. Chauvin kept his knee on Mr. Floyd for more than nine minutes.

The sergeant added that being in a prone position while handcuffed can make it difficult to breathe, saying, “When you add body weight to that, it just increases the possibility of death.”

The latter half of Wednesday’s testimony, which focused on the Minnesota Bureau of Criminal Apprehension’s investigation, seemed to indicate a shift away from questions about whether Mr. Chauvin violated police policy and toward the issue of drugs.

More than 20 witnesses have taken the stand for the prosecution, but the defense has to then call its witnesses, after which the trial can move into closing arguments. A verdict may still be weeks away.

As the trial of Derek Chauvin continued on Wednesday, chalk drawings and signage memorializing George Floyd could be seen on streets and walls of Minneapolis.

A use-of-force expert called by prosecutors testified on Wednesday that Derek Chauvin, the police officer charged with murdering George Floyd, used “deadly force” when it was appropriate to use none.

The expert, Sgt. Jody Stiger, who works with the Los Angeles Police Department Inspector General’s Office, also said that Mr. Chauvin put Mr. Floyd at risk of positional asphyxia, or a deprivation of oxygen. His testimony could corroborate one of the prosecution’s primary assertions: That Mr. Floyd died from asphyxia because Mr. Chauvin knelt on him for more than nine minutes.

Senior Special Agent James D. Reyerson of the Minnesota Bureau of Criminal Apprehension, whose agency investigates police use of force, told jurors about the bureau’s investigation into Mr. Floyd’s death, and said that Mr. Floyd shouted “I ain’t do no drugs,” while he was handcuffed. Here are the highlights from Wednesday.

Sergeant Stiger testified that “no force should have been used” once Mr. Floyd was subdued, handcuffed and facedown on the pavement. “He was in the prone position, he was handcuffed, he was not attempting to resist, he was not attempting to assault the officers — kick, punch, or anything of that nature,” Sergeant Stiger said. The prosecution has argued that Mr. Chauvin’s force continued for far longer than necessary; in all, Mr. Chauvin pinned Mr. Floyd with his knee for about nine and a half minutes.

Responding to questions from the defense, Sergeant Stiger said that Mr. Floyd resisted arrest when the responding officers tried to put him in the back of a squad car. In that moment, Mr. Chauvin would have been justified in using a Taser, Sergeant Stiger said. The defense has suggested that people who do not appear to be dangerous to officers can quickly pose a threat. The line of questioning appeared to be an attempt to establish that Mr. Floyd had been combative at first, and therefore could have become so once again. Sergeant Stiger pushed back on the argument, saying that officers should use force that is necessary for what suspects are doing in the moment, not what they might do later.

Asked to interpret footage from a police body camera, Mr. Reyerson initially said Mr. Floyd appeared to say, “I ate too many drugs.” But in later testimony, Mr Reyerson changed his assessment and said that Mr. Floyd had actually shouted, “I ain’t do no drugs.” His revised judgment could chip away at Mr. Chauvin’s defense, which has tried to argue that Mr. Floyd died from complications of drug use, not the actions of Mr. Chauvin. A toxicology report found methamphetamine and fentanyl in Mr. Floyd’s system. Sergeant Stiger told the jury that he could not make out what Mr. Floyd said in that moment.

Much of Wednesday’s proceedings focused on Mr. Floyd’s drug use. The jury heard testimony from McKenzie Anderson, a forensic scientist with the Minnesota Bureau of Criminal Apprehension who processed the squad car that Mr. Floyd was briefly placed in on the night he died. An initial processing found no drugs in the vehicle, but during a second search requested by Mr. Chauvin’s defense team in January, the team discovered fragments of pills. Judge Peter Cahill has called the oversight “mind-boggling.” Ms. Anderson said she was not looking for pills during the initial search, and simply passed over them. In testing the fragments, Ms. Anderson said a lab found D.N.A. that matched Mr. Floyd’s.

Breahna Giles, a forensic scientist with the Minnesota Bureau of Criminal Apprehension, testified that some of the pills recovered at the scene were tested and found to contain methamphetamine and fentanyl. The pills were marked with letters and numbers that seemed to indicate that they were pharmaceutical-grade Acetaminophen and Oxycodone, though illicit pills are sometimes marked by drug dealers to give the false impression that they came from a pharmacy.

Midway through the second week of the trial of Derek Chauvin, more than 20 witnesses have already taken the stand for the state. Next, the defense will present their witnesses, before the trial moves into closing arguments and, finally, jury deliberation.

Witness testimony is expected to last at least through the end of next week. On Friday, Judge Peter A. Cahill dismissed court early, saying that the trial was ahead of schedule.

Jury selection — eight days of intense questioning to potential jurors about their political biases and views on racism and policing — began on March 9. Ultimately, 12 jury members and two alternates were chosen.

Both sides delivered opening statements on March 29, which were followed by the prosecution calling their witnesses to the stand. Each witness is questioned by the state, then cross-examined by the defense. Questioning goes back and forth between the state and the defense.

Each side submitted a list of potential witnesses to the judge ahead of the trial: The state submitted the names of 363 potential witnesses, and the defense listed 212, but it’s unclear how many will actually appear.

Closing arguments could come as soon as the week after next, then the jury will begin deliberating. The jury can take as long as it needs to deliver a verdict.

It’s a Monday evening in Minneapolis. Police respond to a call about a man who allegedly used a counterfeit $20 bill to buy cigarettes. Seventeen minutes later, the man they are there to investigate lies motionless on the ground, and is pronounced dead shortly after. The man was 46-year-old George Floyd, a bouncer originally from Houston who had lost his job at a restaurant when the coronavirus pandemic hit. Crowd: “No justice, no peace.” Floyd’s death triggered major protests in Minneapolis, and sparked rage across the country. One of the officers involved, Derek Chauvin, has been arrested and charged with second-degree murder. The other three officers have been charged with aiding and abetting murder. The Times analyzed bystander videos, security camera footage and police scanner audio, spoke to witnesses and experts, and reviewed documents released by the authorities to build as comprehensive a picture as possible and better understand how George Floyd died in police custody. The events of May 25 begin here. Floyd is sitting in the driver’s seat of this blue S.U.V. Across the street is a convenience store called Cup Foods. Footage from this restaurant security camera helps us understand what happens next. Note that the timestamp on the camera is 24 minutes fast. At 7:57 p.m., two employees from Cup Foods confront Floyd and his companions about an alleged counterfeit bill he just used in their store to buy cigarettes. They demand the cigarettes back but walk away empty-handed. Four minutes later, they call the police. According to the 911 transcript, an employee says that Floyd used fake bills to buy cigarettes, and that he is “awfully drunk” and “not in control of himself.” Soon, the first police vehicle arrives on the scene. Officers Thomas Lane and J. Alexander Kueng step out of the car and approach the blue S.U.V. Seconds later, Lane pulls his gun. We don’t know exactly why. He orders Floyd to put his hands on the wheel. Lane reholsters the gun, and after about 90 seconds of back and forth, yanks Floyd out of the S.U.V. A man is filming the confrontation from a car parked behind them. The officers cuff Floyd’s hands behind his back. And Kueng walks him to the restaurant wall. “All right, what’s your name?” From the 911 transcript and the footage, we now know three important facts: First, that the police believed they were responding to a man who was drunk and out of control. But second, even though the police were expecting this situation, we can see that Floyd has not acted violently. And third, that he seems to already be in distress. Six minutes into the arrest, the two officers move Floyd back to their vehicle. As the officers approach their car, we can see Floyd fall to the ground. According to the criminal complaints filed against the officers, Floyd says he is claustrophobic and refuses to enter the police car. During the struggle, Floyd appears to turn his head to address the officers multiple times. According to the complaints, he tells them he can’t breathe. Nine minutes into the arrest, the third and final police car arrives on the scene. It’s carrying officers Tou Thao and Derek Chauvin. Both have previous records of complaints brought against them. Thao was once sued for throwing a man to the ground and hitting him. Chauvin has been involved in three police shootings, one of them fatal. Chauvin becomes involved in the struggle to get Floyd into the car. Security camera footage from Cup Foods shows Kueng struggling with Floyd in the backseat while Thao watches. Chauvin pulls him through the back seat and onto the street. We don’t know why. Floyd is now lying on the pavement, face down. That’s when two witnesses begin filming, almost simultaneously. The footage from the first witness shows us that all four officers are now gathered around Floyd. It’s the first moment when we can clearly see that Floyd is face down on the ground, with three officers applying pressure to his neck, torso and legs. At 8:20 p.m., we hear Floyd’s voice for the first time. The video stops when Lane appears to tell the person filming to walk away. “Get off to the sidewalk, please. One side or the other, please.” The officers radio a Code 2, a call for non-emergency medical assistance, reporting an injury to Floyd’s mouth. In the background, we can hear Floyd struggling. The call is quickly upgraded to a Code 3, a call for emergency medical assistance. By now another bystander, 17-year-old Darnella Frazier, is filming from a different angle. Her footage shows that despite calls for medical help, Chauvin keeps Floyd pinned down for another seven minutes. We can’t see whether Kueng and Lane are still applying pressure. Floyd: [gasping] Officer: “What do you want?” Bystander: “I’ve been —” Floyd: [gasping] In the two videos, Floyd can be heard telling officers that he can’t breathe at least 16 times in less than five minutes. Bystander: “You having fun?” But Chauvin never takes his knee off of Floyd, even as his eyes close and he appears to go unconscious. Bystander: “Bro.” According to medical and policing experts, these four police officers are committing a series of actions that violate policies, and in this case, turn fatal. They’ve kept Floyd lying face down, applying pressure for at least five minutes. This combined action is likely compressing his chest and making it impossible to breathe. Chauvin is pushing his knee into Floyd’s neck, a move banned by most police departments. Minneapolis Police Department policy states an officer can only do this if someone is, quote, “actively resisting.” And even though the officers call for medical assistance, they take no action to treat Floyd on their own while waiting for the ambulance to arrive. Officer: “Get back on the sidewalk.” According to the complaints against the officers, Lane asks him twice if they should roll Floyd onto his side. Chauvin says no. Twenty minutes into the arrest, an ambulance arrives on the scene. Bystander: “Get off of his neck!” Bystander: “He’s still on him?” The E.M.T.s check Floyd’s pulse. Bystander: “Are you serious?” Chauvin keeps his knee on Floyd’s neck for almost another whole minute, even though Floyd appears completely unresponsive. He only gets off once the E.M.T.s tell him to. Chauvin kept his knee on Floyd’s neck for over eight minutes, according to our review of the video evidence. Floyd is loaded into the ambulance. The ambulance leaves the scene, possibly because a crowd is forming. But the E.M.T.s call for additional medical help from the fire department. But when the engine arrives, the officers give them, quote, “no clear info on Floyd or his whereabouts,” according to a fire department incident report. This delays their ability to help the paramedics. Meanwhile, Floyd is going into cardiac arrest. It takes the engine five minutes to reach Floyd in the ambulance. He’s pronounced dead at a nearby hospital around 9:25 p.m. Preliminary autopsies conducted by the state and Floyd’s family both ruled his death a homicide. The widely circulated arrest videos don’t paint the entire picture of what happened to George Floyd. Crowd: “Floyd! Floyd!” Additional video and audio from the body cameras of the key officers would reveal more about why the struggle began and how it escalated. The city quickly fired all four officers. And Chauvin has been charged with second degree murder. Thomas Lane, J. Alexander Kueng and Tou Thao were charged with aiding and abetting murder. But outrage over George Floyd’s death has only spread further and further across the United States.

On May 25, Minneapolis police officers arrested George Floyd, a 46-year-old Black man, after a convenience store employee called 911 to report that Mr. Floyd had bought cigarettes with a counterfeit $20 bill. Seventeen minutes after the first squad car arrived at the scene, Mr. Floyd was unconscious and pinned beneath three police officers, showing no signs of life.

By combining videos from bystanders and security cameras, reviewing official documents and consulting experts, The New York Times reconstructed in detail the minutes leading to Mr. Floyd’s death. Our video shows officers taking a series of actions that violated the policies of the Minneapolis Police Department and turned fatal, leaving Mr. Floyd unable to breathe, even as he and onlookers called out for help.

The trial of Derek Chauvin in the death of George Floyd is unusual for many reasons: It is being livestreamed from Minneapolis, attendance is severely limited because of the coronavirus and the public’s interest in the case may make this one of the highest-profile trials in recent memory.

The trial can be watched on, via a livestream provided by Court TV, which is also airing the trial in full. Witness testimony and lawyers’ presentations of evidence should last several weeks before the jury begins to deliberate over the verdict.

Among the people allowed in the courtroom, on the 18th floor of the Hennepin County Government Center, are: the judge, jurors, witnesses, court staff, lawyers, Mr. Chauvin and only a handful of spectators.

The judge, Peter A. Cahill, wrote in an order on March 1 that only one member of Mr. Floyd’s family and one member of Mr. Chauvin’s family would be allowed in the room at any time. Two seats are reserved for reporters, and various journalists, including from The New York Times, are rotating throughout the trial.

The lawyers, spectators, jurors and witnesses are required to wear masks when they are not speaking. Spectators are prohibited from having any visible images, logos, letters or numbers on their masks or clothing, according to Judge Cahill’s order.


News – Derek Chauvin Trial Live Updates: George Floyd Showed Signs of Brain Injury 4 Minutes Before Officer Relented, Expert Says