Witnesses addressed two key issues: What caused George Floyd’s death, and whether Mr. Chauvin violated police policies on the use of force.

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The first week of the Derek Chauvin trial was marked by emotional accounts from bystanders who witnessed the nine and a half minutes that the police pinned George Floyd to the ground. But the second week struck a different chord, highlighting testimony from medical and law enforcement experts that centered on the conduct of Mr. Chauvin and the cause of Mr. Floyd’s death.

Those witnesses hit on the key issues of the trial: what exactly killed Mr. Floyd, and whether Mr. Chauvin violated police policies on use of force. The answers to those two questions will be crucial for Mr. Chauvin, the former police officer charged with murdering Mr. Floyd in Minneapolis last May.

Several medical witnesses testified that Mr. Floyd died from a deprivation of oxygen — contradicting claims by the defense lawyer, Eric J. Nelson, who has sought to tie Mr. Floyd’s death to complications from drug use and a heart condition. Law enforcement officials, including the chief of the Minneapolis Police Department, said Mr. Chauvin violated police policy when he used his knee to keep Mr. Floyd pinned to the street.

Here are five key takeaways from the second week of the trial.

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Chief Medaria Arradondo testified Monday that the former officer Derek Chauvin should have halted his use of force to restrain George Floyd after Mr. Floyd had stopped resisting.Still image, via Court TV

On Monday, Chief Medaria Arradondo of the Minneapolis Police Department said Mr. Chauvin “absolutely” violated the department’s policies during the arrest. His statements represented an unusual rebuke of a police officer by an acting chief.

“Once Mr. Floyd had stopped resisting, and certainly once he was in distress and trying to verbalize that, that should have stopped,” Chief Arradondo said. The chief’s statement was one of the most clear-cut and significant on the issue of Mr. Chauvin’s use of force, though several other witnesses also suggested that Mr. Chauvin acted outside the bounds of normal policing.

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Officer Nicole Mackenzie, who trains Minneapolis police officers on providing medical care, said at the trial of Derek Chauvin that bystanders could make it easier for officers to miss signs that a detainee is in distress.Still image, via Court TV

Still, Mr. Nelson, Mr. Chauvin’s lawyer, may have made some headway with other witnesses on the question of force. Officer Nicole Mackenzie, the medical support coordinator for the Minneapolis Police Department, agreed with Mr. Nelson’s assertion that a crowd of vocal bystanders could make it difficult for an officer to render medical aid during an arrest. And Lt. Johnny Mercil, a veteran of the Minneapolis Police Department and a use-of-force instructor, also said that hostile bystanders can raise alarm with officers.

Mr. Nelson has suggested throughout the trial that the crowd of bystanders outside the Cup Foods convenience store, some of whom yelled at Mr. Chauvin during the arrest, may have hindered the former officer from providing help once Mr. Floyd became unresponsive.

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Los Angeles Police Sgt. Jody Stiger, a use-of-force expert, told jurors on Tuesday that the former police officer Derek Chauvin used excessive force on George Floyd during his arrest.Still image, via Court TV

Sgt. Jody Stiger, who works with the Los Angeles Police Department Inspector General’s Office, continued to explore the use-of-force issue by saying that Mr. Chauvin used “deadly force” when he should have used none. He also teed up another aspect of the trial that came into focus later in the week: whether Mr. Floyd’s death was caused by “asphyxia,” or a lack of oxygen.

“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. Responding to questions from the defense, Sergeant Stiger said that Mr. Floyd resisted arrest when the officers tried to place him in the back of a squad car. In those early moments of the arrest, Mr. Chauvin would have been justified if he had decided to use a Taser, Sergeant Stiger said.

The defense has argued that people who do not appear to be dangerous to officers can quickly pose a threat. The sergeant pushed back on that argument, saying that officers should use force that is necessary for what suspects are doing in the moment, not what they might do later.

Mr. Floyd’s drug use was a recurring point of discussion throughout the week. On Wednesday, 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 inspection 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. In testing the fragments, Ms. Anderson said a lab found DNA 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 found to contain methamphetamine and fentanyl. Mr. Chauvin’s defense has suggested that Mr. Floyd died from complications of drug use. Later in the week, the medical examiner who performed the official autopsy of Mr. Floyd said he found no fragments of pills in Mr. Floyd’s stomach contents.

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A veteran pulmonologist Dr. Martin J. Tobin testified 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.Still image, via Court TV

Two medical witnesses on Thursday testified that they saw no evidence that Mr. Floyd died from a drug overdose. The first, Dr. Martin J. Tobin, a pulmonologist and critical care physician from the Chicago area, said that any normal person could have died from being pinned under Mr. Chauvin’s knee for nine and a half minutes.

His testimony gave a moment-by-moment breakdown of the arrest of Mr. Floyd, identifying what he believed to be “the moment the life goes out of his body.” Responding to Mr. Nelson’s suggestion that Mr. Floyd died from complications of fentanyl use — a toxicology report found fentanyl and methamphetamine in his system — Dr. Tobin said Mr. Floyd’s behavior did not correspond with that of a person who was overdosing.

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Dr. Martin Tobin, an expert pulmonologist, said that George 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, despite the argument by Derek Chauvin’s legal team.Still image, via Court TV

He also pushed back on the idea that simply because Mr. Floyd was speaking, he was getting enough oxygen. Dr. Tobin said that a person might be taking in enough oxygen to speak but not enough to survive. The person can be alive and talking one moment, and dead just seconds later, he said. Dr. Bill Smock, the surgeon for the Louisville Metropolitan Police Department, also testified, saying he saw no evidence of an overdose.

“That is not a fentanyl overdose,” Dr. Smock said. “That is somebody begging to breathe.”

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Dr. Bill Smock, a surgeon for the Louisville Metropolitan Police Department, testified on Thursday that he had found no evidence to support claims that George Floyd died of an overdose.

The second week ended with testimony from Dr. Andrew Baker, the Hennepin County medical examiner who performed the official autopsy of George Floyd. Dr. Baker testified that while drug use and a heart condition contributed to Mr. Floyd’s death, police restraint was the main cause.

Leading up to the trial, Dr. Baker had made several statements that could have complicated the arguments of the prosecution, particularly in relation to Mr. Floyd’s drug use. During testimony on Friday, he said that the level of fentanyl found in Mr. Floyd’s system could have been fatal for some people.

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Dr. Andrew Baker, the Hennepin County medical examiner who performed the initial autopsy of George Floyd, said that there were contributing factors to Mr. Floyd’s death but that ultimately it was homicide.Still image, via Court TV

Still, Dr. Baker said that, in Mr. Floyd’s case, it was less likely than other potential causes of death. He added that Mr. Floyd had an enlarged heart for his size, which would require more oxygen to pump blood through his body. High-intensity situations — like the one Mr. Floyd experienced during his arrest — could exacerbate that problem.

“In my opinion, the law enforcement subdual, restraint and the neck compression was just more than Mr. Floyd could take by virtue of those heart conditions,” he said.