The Minneapolis police chief testified Monday that now-fired Officer Derek Chauvin violated departmental policy in pinning his knee on George Floyd’s neck and keeping him down after Floyd had stopped resisting and was in distress.

Continuing to kneel on Floyd's neck once he was handcuffed behind his back and lying on his stomach was “in no way, shape or form” part of department policy or training, "and it is certainly not part of our ethics or our values,” Police Chief Medaria Arradondo said.

Arradondo, the city’s first Black chief, fired Chauvin and three other officers the day after Floyd’s death last May, and in June called it “murder.”

His testimony came after the emergency room doctor who pronounced Floyd dead testified that he theorized at the time that Floyd's heart most likely stopped because of a lack of oxygen.

Dr. Bradford Langenfeld, who was a senior resident on duty that night at Hennepin County Medical Center and tried to resuscitate Floyd, took the stand at the beginning of Week Two at Chauvin’s murder trial, as prosecutors sought to establish that it was Chauvin’s knee on the Black man’s neck that killed him.

Langenfeld said Floyd’s heart had stopped by the time he arrived at the hospital. The doctor 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.

Under questioning by prosecutor Jerry Blackwell, Langenfeld said that based on the information he had, it was “more likely than the other possibilities” that Floyd's cardiac arrest - the stopping of his heart - was caused by asphyxia, or insufficient oxygen.

Chauvin, 45, is charged with murder and manslaughter in Floyd's death May 25. The white officer is accused of digging his knee into the 46-year-old man's neck for 9 minutes, 29 seconds, outside a corner market, where Floyd had been accused of trying to pass a counterfeit $20 bill for a pack of cigarettes.

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.

Chauvin attorney Eric Nelson questioned Langenfeld about whether some drugs can cause hypoxia, or insufficient oxygen. The doctor acknowledged that fentanyl and methamphetamine, both of which were found in Floyd's body, can do so.

The county medical examiner's office ultimately classified Floyd's death a homicide - that is, a death at the hands of someone else.

The full report said Floyd died of “cardiopulmonary arrest, complicating law enforcement subdual, restraint, and neck compression.” A summary report listed fentanyl intoxication and recent methamphetamine use under “other significant conditions” but not under “cause of death.”

Under cross-examination from Nelson, Langenfeld said Floyd’s carbon dioxide levels were more than twice as high as levels in a healthy person, and he agreed that that could be attributed to a respiratory problem. But on questioning from the prosecutor, the doctor said the high levels were also consistent with cardiac arrest.

Langenfeld also testified that neither he nor paramedics administered a drug that would reverse the effects of an opioid overdose. The doctor said giving Narcan once a patient is in cardiac arrest would provide no benefit.

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.

Langenfeld said that “any amount of time” 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 Chauvin’s training in the use of force.

Arradondo also testified about police policy that dictates that whenever it is reasonable to do so, officers must use tactics to deescalate a situation so as to avoid or minimize the use of force.

Prosecutor Steve Schleicher noted that while some people may become more dangerous under the influence of drugs or alcohol, some may actually be “more vulnerable.” Arradondo agreed and acknowledged that this must also be taken into consideration when officers decide to use force.

“It’s recognizing that when we get the call from our communities, it may not often be their best day, and they may be experiencing something that’s very traumatic,” the chief said.

Before he was pinned to the ground, a handcuffed and frantic Floyd struggled with police who were trying to put him in a squad car, saying he was claustrophobic.

Arradondo said officers are trained in basic first aid, including chest compressions, and department policy requires them to request medical assistance and provide necessary aid as soon as possible before paramedics arrive.

Officers' first aid training is “very vital because those seconds are vital," Arradondo said, adding: “And so we absolutely have a duty to render that."

Officers kept restraining Floyd - with Chauvin kneeling on his neck, another kneeling on Floyd’s back and a third holding his feet - until the ambulance arrived, even after he became unresponsive, according to testimony and video footage.

One officer asked twice if they should roll Floyd on his side to aid his breathing, and later said calmly that he thought Floyd was passing out. Another checked Floyd’s wrist for a pulse and said he couldn’t find one.

The officers also rebuffed offers of help from an off-duty Minneapolis firefighter who wanted to administer aid or tell officers how to do it.

The city moved soon after Floyd's death to ban police chokeholds and neck restraints. Arradondo and Mayor Jacob Frey also made several policy changes, including expanding requirements for reporting use-of-force incidents and documenting attempts to de-escalate situations.

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 to the ground.

“Totally unnecessary,” Lt. Richard Zimmerman, the longest-tenured officer on the force, testified Friday.