MINNEAPOLIS — The police officer had seen hundreds of crime scenes, interviewed scores of witnesses and made his share of arrests over more than 35 years working cases in Minneapolis.

But when Lt. Richard Zimmerman watched a video of one of his colleagues kneeling on George Floyd's neck, he saw what he described in a courtroom on Friday as a "totally unnecessary" violation of department policy.

"Pulling him down to the ground facedown and putting your knee on a neck for that amount of time, it's just uncalled-for," testified Lieutenant Zimmerman, who is the longest-serving officer on the Minneapolis police force. His comments came at the end of the first week in the trial of Derek Chauvin, the former police officer charged with murdering Mr. Floyd along a Minneapolis street last May.

Police officers have often been accused of sticking together on questions of misconduct — avoiding breaking a so-called blue wall of silence — so the sworn testimony against Mr. Chauvin by a high-ranking officer was all the more extraordinary.

Only a day earlier, another police official, who had directly supervised Mr. Chauvin, testified that Mr. Chauvin and two other officers should have stopped restraining Mr. Floyd sooner. And in the coming week the city's police chief, Medaria Arradondo, who has called Mr. Floyd's death a "murder," is also expected to condemn Mr. Chauvin's actions from the witness stand.

All of it seemed to undermine an assertion that Mr. Chauvin's lawyers have made a central point in the former officer's defense — that Mr. Chauvin's behavior as he arrested Mr. Floyd was within the bounds of his police training.

Lieutenant Zimmerman, 62, who peppered his testimony with references to his long career in law enforcement and concurred with a lawyer's suggestion that he had joined the department as an "old-school cop" in 1985, was unwavering in his assessment of Mr. Chauvin's actions. He often turned to speak directly to the 12 jurors who are expected to decide the verdict.

"If you're kneeling on a person's neck, that can kill him," said Lieutenant Zimmerman, who has led the Minneapolis department's homicide unit since 2008. Officers are supposed to turn people onto their sides or sit them up once they are restrained, he said, because leaving them in prone positions can make it hard to breathe.

Mr. Chauvin and two other police officers had continued to pin Mr. Floyd, who was handcuffed, against the ground after he was no longer responsive. That decision, Lieutenant Zimmerman suggested, meant the officers had violated their duty to take care of someone in their custody.

"His safety is your responsibility," he told the court. "His well-being is your responsibility."

Lieutenant Zimmerman testified on the fifth day of the high-profile trial, which began 10 months after Mr. Floyd's death set off global protests over racism and police abuse. Jurors have heard from more than a dozen witnesses, including the teenager who filmed the widely viewed video of Mr. Chauvin kneeling on Mr. Floyd for more than nine minutes, the convenience store clerkwho told his manager that Mr. Floyd had paid for cigarettes using a fake $20 bill, and Mr. Floyd's girlfriend, who described their shared struggle with opioid addiction.

The testimony from police officials, though, marked a shift to a different phase of the case: Prosecutors have said they will show that Mr. Chauvin's actions were unusually brutal — and amounted to a crime.

In cross-examining Lieutenant Zimmerman, Eric J. Nelson, the lawyer for Mr. Chauvin, suggested that the lieutenant's experience on the streets might be dated. Lieutenant Zimmerman had not regularly patrolled the streets as a uniformed officer since 1993, Mr. Nelson noted, offering that he might no longer be familiar with the force needed. At one point, Mr. Nelson asked Lieutenant Zimmerman when he had last gotten into a fight with someone while on duty; 2018, the lieutenant answered.

Under questioning, Lieutenant Zimmerman acknowledged that people sometimes become more combative when revived after a period of unconsciousness and said that police officers had been trained to kneel on people's shoulders, in some circumstances, while handcuffing them.

He said that once people are handcuffed, they usually present only a minor threat, though they can still be combative and try to hurt officers, such as by kicking them.

"Once a person is cuffed, the threat level goes down all the way," Lieutenant Zimmerman said. "They're cuffed; how can they really hurt you?"

In body camera footage shown to jurors, Mr. Floyd can be seen in handcuffs when Mr. Chauvin first kneels on his neck. Paramedics testified this week that his heart had stopped by the time they arrived.

All of the witnesses so far have been called by prosecutors, who are expected to call more witnesses next week, after which Mr. Chauvin's defense team can begin laying out its arguments in earnest.

In opening statements, the defense has suggested that Mr. Floyd's death, which the county medical examiner ruled a homicide, may actually have been caused by the fentanyl and methamphetamine found in his system. Mr. Chauvin's defense team has also indicated that he was following procedures that he had learned in his training.

Yet Sgt. David Pleoger, who was Mr. Chauvin's supervisor and who testified for the prosecution on Thursday, said that officers should have stopped holding Mr. Floyd down once he became unresponsive.

He also said that Mr. Chauvin had at first not divulged that he knelt on Mr. Floyd. In an initial phone call with Sergeant Pleoger, minutes after Mr. Floyd was taken to a hospital, Mr. Chauvin said that he and other officers "had to hold the guy down" because Mr. Floyd would not stay in the back of a police car and was "going crazy." About 30 minutes later, when officials learned that Mr. Floyd's condition was grave, Sergeant Pleoger said, Mr. Chauvin acknowledged that he had pressed on Mr. Floyd's neck.

Lieutenant Zimmerman's courtroom testimony was not the first time he had rebuked Mr. Chauvin's conduct, nor the first time he had testified against a fellow officer. In a 2019 murder case against Mohamed Noor, a Minneapolis police officer who fatally shot an unarmed woman, Lieutenant Zimmerman testified that the scene of the shooting was well-lit, contradicting claims by Mr. Noor's lawyers that it had been difficult to see. Mr. Noor was convicted of third-degree murder, the less serious of two murder charges that Mr. Chauvin faces.

After Mr. Floyd's death and the unrest that followed, Lieutenant Zimmerman was among 14 veteran police officers who published a public letter condemning Mr. Chauvin. He had "failed as a human and stripped George Floyd of his dignity and life," the officers wrote, adding that a "vast majority" of police officers felt the same. The officers said in the letter, which was addressed to the citizens of Minneapolis, that they hoped to regain the public's trust.

"This is not who we are," they wrote.

New York Times, By Tim Arango and Nicholas Bogel-Burroughs, April 2, 2021

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April 3, 2021

Voices4America Post Script.Another police office -the longest serving Minneapolis officer-said Chauvin exercised unnecessary force against #GeorgeFloyd. The prosecution has all it needs for its summary appeal to the jury. #BlackLivesMatte