The emergency room doctor who pronounced George Floyd dead after trying to resuscitate him testified Monday that Floyd’s heart had stopped when he arrived. Dr. Bradford Langenfeld, who was a senior resident on duty that night at Hennepin County Medical Center, said that he was not told of any efforts at the scene by bystanders or police to resuscitate Floyd but that paramedics told him they had tried for about 30 minutes.

He took the stand at the beginning of Week Two at former Minneapolis Officer Derek Chauvin’s murder trial, as prosecutors sought to establish that it was Chauvin’s knee on the Black man’s neck that killed him last May.

Langenfeld said that every minute a patient spends in cardiac arrest without immediate CPR decreases the chance of a good outcome. He said there is an approximately 10% to 15% decrease in survival for every minute that CPR is not administered.

Prosecutors in the second week of the trial are also expected to zero in on the officer’s training in the use of force. The first week of the case was dominated by emotional testimony from eyewitnesses and devastating video of Floyd’s arrest.

Derek Chauvin, 45, is charged with murder and manslaughter in Floyd’s death May 25. The white officer is accused of pinning his knee on the 46-year-old man’s neck for 9 minutes, 29 seconds, as Floyd lay face-down in handcuffs outside a corner market.

The defense argues that Chauvin did what he was trained to do and that Floyd’s use of illegal drugs and his underlying health conditions caused his death.

Floyd’s treatment by police was captured on widely seen bystander video that sparked protests that rocked Minneapolis and quickly spread to other U.S. cities and beyond and descended into violence in some cases.

Minneapolis Police Chief Medaria Arradondo was also expected to testify Monday. Arradondo, the city’s first Black chief, fired Chauvin and three other officers the day after Floyd’s death, and in June called it “murder.”

“Mr. George Floyd’s tragic death was not due to a lack of training — the training was there,” Arradondo said then. “Chauvin knew what he was doing.”

The city moved soon after Floyd’s death to ban police chokeholds and neck restraints. Arradondo and Mayor Jacob Frey also have made several policy changes, including expanding requirements for reporting use-of-force incidents and documenting their attempts to de-escalate situations even when force isn’t used.

Prosecutors have already called supervisory officers to build the case that Chauvin improperly restrained Floyd. A duty sergeant and a lieutenant who leads the homicide division both questioned Chauvin’s actions in pinning Floyd after officers responded to a report that Floyd had passed a counterfeit $20 bill.

“Totally unnecessary,” Lt. Richard Zimmerman, the longest-tenured officer on the force, testified Friday. He said once Floyd was handcuffed, he saw “no reason for why the officers felt they were in danger, if that’s what they felt, and that’s what they would have to feel to be able to use that kind of force.”