The testimony comes on the fifth day of a murder trial against ex-US police officer Derek Chauvin.
The top homicide investigator in the US city of Minneapolis has said former police officer Derek Chauvin used “totally unnecessary” force when arresting George Floyd.
Richard Zimmerman was testifying on the fifth day of Mr Chauvin’s murder trial.
White officer Mr Chauvin was filmed kneeling on African-American Mr Floyd’s neck for more than nine minutes last May, sparking global protests.
The 45-year-old has denied charges of murder and manslaughter.
In his testimony on Friday, Lieutenant Zimmerman said he arrived on the scene after Mr Floyd’s death in order to help ensure evidence was secured and witnesses were found.
He said officers were responsible for the safety and wellbeing of anyone they arrested.
“Totally unnecessary,” he said, when asked about Mr Chauvin’s actions. “If your knee is on a person’s neck, that can kill them.”
He added that he could see no reason for Mr Chauvin to keep his knee on Mr Floyd for more than nine minutes.
“First of all, pulling him down to the ground facedown and putting your knee on a neck for that amount of time is just uncalled for,” he said.
“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.”
Paramedics, bystanders and Mr Floyd’s girlfriend are among those who have taken the stand since the trial began on Monday.
At its opening, prosecutor Jerry Blackwell said Mr Chauvin had “betrayed his badge” and claimed he used “excessive and unreasonable force”.
Defence lawyers have indicated they will argue that Mr Floyd died of an overdose and poor health, and the force used in his arrest was reasonable.
Footage from both witnesses’ mobile phones and the police officers’ bodycams have been shown to the jury at length.
In one video, Mr Floyd could be heard pleading with officers, saying: “I’m not a bad guy”.
In another, Mr Chauvin is heard telling a bystander: “We had to control this guy because he’s a sizeable guy. It looks like he’s probably on something.”
The court also examined security camera footage of Mr Floyd inside a grocery store, where he allegedly paid for a packet of cigarettes with a counterfeit note, leading to his arrest.
Shop employee Christopher Martin said Mr Floyd “didn’t seem to know it was a fake note”, adding that he watched the arrest with “disbelief and guilt”.
Mr Floyd’s girlfriend, Courteney Ross, gave emotional testimony, in which she described their first kiss and struggle with opioid addiction.
Two paramedics later said Mr Floyd had no pulse and did not appear to be breathing when they arrived at the scene.
Jurors have also heard from David Pleoger, the supervising police officer on duty that day, who said he only learned later in the evening that Mr Chauvin had restrained Mr Floyd by kneeling on his neck.
He told the court that an officer should stop using the knee as a restraint once “you get control of the situation” and the suspect is in handcuffs.
Defence lawyer Eric Nelson sought to play down the significance of his testimony by saying Mr Pleoger had “not reviewed the entirety of the evidence in this particular case”.
Throughout the testimony, Mr Chauvin has been taking almost constant notes.
Here in Minneapolis, the trial is everywhere. It is playing on car radios, in barber shops, supermarkets and on mobile phones.
George Floyd’s death has shaken this city – and the country – to its core.
Nationwide, the percentage of people who are concerned about race relations has hit a record high: 48%, according to a poll taken in March. That is most likely because of Mr Floyd’s death and the unrest that followed.
People here are waiting anxiously for the verdict and say they hope justice will be served. Even those who do not normally follow the news are tracking this momentous trial.
I asked one man who was sat near a shopping mall what he thought about the trial. He looked up from his phone: “Guilty,” he barked, then went back to scrolling.
Soon the jury will decide, either supporting this man’s view or not. Until then, he and others are left waiting.
Footage of Mr Chauvin kneeling on Mr Floyd’s neck was viewed around the world and sparked mass demonstrations against racism and police brutality.
Former US President Barack Obama said the protests represented a “genuine and legitimate frustration over a decades-long failure to reform police practices and the broader criminal justice system”.
Police officers have rarely been convicted – if they are charged at all – for deaths that occur in custody, and the verdict in this trial is being widely seen as an indication of how the US legal system will treat such cases in future.