Derek Chauvin Trial Live Updates: Medical Experts Take the Stand

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

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.

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.

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 may have been justified if he had decided to use 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 face down 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.

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. Chauvin 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.

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.

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.

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.