A Minneapolis homicide lieutenant with over 40 years of experience said Friday that Derek Chauvin's use of force on George Floyd was "totally unnecessary," testifying that "if your knee is on a person's neck, that can kill him."

Why it matters: Though his role is in investigation and not on patrol — something the defense emphasized — media commentators called it the most compelling testimony yet from law enforcement who have taken the stand at former Minneapolis police officer Derek Chauvin's murder trial.

Driving the news: The night of the incident, Richard Zimmerman was called to the scene where George Floyd died after Chauvin kneeled on his neck for over nine minutes. Having reviewed footage, he testified that Chauvin's use of force — which he considered "the top tier, the deadly force" — was "uncalled for."

• Once a person is handcuffed, the threat level goes down "all the way" and officers are trained to use restraint, Zimmerman said.
• The goal is to get people out of the prone position as soon as possible because it's dangerous, he added, noting that the handcuffs constrict a person's breathing because they "stretch the muscles through your chest."
• If someone becomes less combative, an officer should try to calm them down, Zimmerman said. That person, their wellbeing and their safety become "your responsibility," he emphasized.

What they're saying: "Pulling him down to the ground face down, and putting your knee on a neck for that amount of time is just uncalled for," Zimmerman testified. "I saw no reason why the officers felt they were in danger if that's what they felt. And that's what they would have to have felt to be able to use that kind of force."

Between the lines: Several jurors started taking notes — more so than usual — during the line of questioning, NBC News reports.

The big picture: Zimmerman's testimony closed out the first week of what is expected to be a monthlong trial. Prosecutors are seeking Chauvin's conviction on murder and manslaughter charges.

• A paramedic told the jury Thursday that when he arrived at the scene, he assumed "there was potentially some struggle still because [multiple officers] were still on top of him."

Recap of the week:

• George Floyd's girlfriend recounts how they met, struggled with opioids
• Witness breaks down after George Floyd video: "I understand him"
• Cashier says he felt "guilt" for accepting George Floyd's fake bill
• Off-duty EMT: I was "desperate" to help Floyd but "officers didn't let me into the scene"
• Witness who recorded George Floyd's death says she stays up at night "apologizing" to him
• 911 dispatcher testimony at Chauvin trial: "Something was not right"
• Prosecutors play 911 call at Derek Chauvin trial