By Ray Sanchez and Aaron Cooper, CNN

(CNN) -- The Minneapolis Police Department's top homicide detective testified at the murder trial of the former officer who knelt on George Floyd's neck last May that "if your knee is on someone's neck -- that could kill them."

Lt. Richard Zimmerman, head of the homicide division for more than 12 years, testified Friday that Derek Chauvin's actions violated policy and that police are not trained to kneel on a person's neck.

"Once the person is cuffed, the threat level goes down all the way," the lieutenant said on the stand.

"How can that person hurt you?" he asked, adding that "you getting injured is way down." Keeping the person handcuffed and in a prone position "restricts their breathing," he said.

Asked by prosecutor Matthew Frank if he was ever trained to kneel on a person, Zimmerman said no.

"Because if your knee is on someone's neck -- that could kill them," Zimmerman said.

Chauvin at that point raised his head at the defense table and shot a look at Zimmerman.

The potentially devastating testimony came on the closely-watched trial's abbreviated fifth day of testimony. Judge Peter Cahill said he would send the jury home early because the trial was ahead of schedule.

Zimmerman said Chauvin's actions were "totally unnecessary."

"You need to get them off their chest," the veteran investigator said at one point. "If you're lying on your chest, that's constricting your breathing even more."

Earlier, jurors heard testimony from the sergeant who secured the area shortly after Chauvin knelt on Floyd's neck.

Sgt. Jon Edwards, a 14-year police veteran, said he arrived at the scene of a "possible critical incident" a little after 9:30 p.m. and had other officers canvass the area for potential witnesses.

At the scene, Edwards said, he asked two officers -- J. Alexander Kueng and Thomas Lane -- to activate their body-worn cameras. Both officers were later charged with aiding and abetting second-degree murder and aiding and abetting second-degree manslaughter.

Zimmerman arrived at the Chicago Avenue scene shortly before 10 p.m. Kueng and Lane were taken to City Hall as part of a critical incident investigation, according to Edwards.

On Thursday, jurors heard Chauvin's perspective in the minutes after Floyd's limp body was taken away in an ambulance. It was the second time they heard his take on the events of that day. A clip from Chauvin's body camera, viewed by jurors on Wednesday, showed Chauvin defending his actions to a bystander.

Then, a call to Sgt. David Pleoger, his supervisor at the time, was captured on body camera footage and played during Pleoger's testimony on Thursday. Chauvin made the call shortly after kneeling on Floyd's neck for more than nine minutes on May 25 to explain his version of what happened.

"I was just going to call and have you come out to our scene here," Chauvin told Pleoger. "We just had to hold a guy down. He was going crazy. He wouldn't ... he wouldn't go in the back of the squad -- "

The video ends but in the rest of the call, Chauvin said Floyd had a medical emergency after struggling with officers trying to put him into a car, according to Pleoger. Chauvin did not mention holding his knee down on Floyd's neck and back, Pleoger said.

Pleoger drove to the scene and asked officers to speak to witnesses. "We can try, but they're all pretty hostile," Chauvin responded.

Later that night, at the Hennepin County Medical Center, Chauvin told his supervisor that he had knelt on Floyd's neck, Pleoger told the jury.

The clip from Chauvin's body camera played for the jury on Wednesday also showed him defending his actions to a bystander who called him out for his treatment of Floyd.

"That's one person's opinion," Chauvin responded as he got into his squad car. "We had to control this guy because he's a sizable guy. It looks like he's probably on something."

His version of the encounter is contradicted by videos showing Chauvin kneeling on Floyd, who was handcuffed after he had passed out. Prosecutors said he knelt on Floyd for 3 minutes and 51 seconds during which Floyd was non-responsive.

Chauvin has pleaded not guilty to charges of second-degree murder, third-degree murder and second-degree manslaughter. The defendant, in a suit and tie, has sat at the defense table, taking notes on a large legal pad.

Earlier Thursday, Floyd's girlfriend spoke about Floyd's struggles with opioid addiction, and a pair of first responders testified that Floyd appeared dead when they arrived.

Supervisor says Chauvin's use of force should have ended earlier

Pleoger's testimony centered on police protocols for the use of force. Officers can use force in certain circumstances, but that force should stop once the person is under control.

Pleoger testified that his review of bodycam footage showed Chauvin's use of force should have ended earlier.

"When Mr. Floyd was no longer offering up any resistance to the officers, they could have ended the restraint," he said. "It would be reasonable to put a knee on someone's neck until they were not resisting anymore, but it should stop when they are no longer combative."

He said officers are required to call an ambulance and provide emergency aid while waiting for the ambulance. Restrained persons should be put on their side to help their breathing.

Pleoger, on cross-examination, said he had not conducted a formal use of force review because the death investigation moved up the chain of command.

'I thought he was dead'

A pair of Hennepin County paramedics said Floyd was unresponsive, not breathing and had no pulse when they arrived on the scene.

"In layman terms, I thought he was dead," paramedic Derek Smith said.

Smith and his partner Seth Bravinder were first called to the scene as a non-emergency Code 2 for a mouth injury. A minute and a half later, the call was upgraded to a Code 3 -- meaning the ambulance uses lights and sirens.

Floyd did not appear to be breathing or moving. Smith checked Floyd's pulse and pupils while Chauvin kept his knee on him. The paramedic said he believed his heart had stopped. Bravinder motioned for Chauvin to lift his knee off Floyd to get him on a stretcher and in the ambulance.

Bravinder said they were concerned about the crowd of bystanders.

An officer climbed into the ambulance and helped with chest compressions. Smith removed Floyd's handcuffs, he testified. Bravinder stopped the ambulance at one point to assist in Floyd's treatment, he testified.

In the ambulance, however, Floyd had flatlined -- meaning his heart showed no activity. Attempts to restart his heart with chest compressions, establishing an airway and electric shock failed. They dropped him off at the hospital.

Fire Department Capt. Jeremy Norton testified that no one ever found a pulse in Floyd's body. He later reported the incident to superiors in the Fire Department because it involved the death of someone in police custody and an off-duty firefighter was a witness.

Floyd and girlfriend struggled with opioid addiction

Courteney Ross, 45, told the jury she met Floyd in August 2017. He worked as a security guard at the Salvation Army.

Floyd worked out every day and never complained of shortness of breath, she said. He was a mama's boy who was a "shell of himself" after his mother's death in 2018.

They both struggled with addiction to opioids, she testified. They were prescribed opioid painkillers to treat chronic pain, which ultimately led to an addiction and their use of street drugs, she testified.

In March 2020, she found Floyd doubled over in pain and took him to the emergency room, she testified. He had overdosed, she said. Ross said she believed he had started using again in May 2020.

In opening statements, prosecutors acknowledged Floyd's history of opioid addiction but said it was irrelevant to his death last May. But defense attorney Eric Nelson argued that Floyd's true cause of death was drug use and several preexisting health issues.