Officer Derek Chauvin had his knee on George Floyd’s neck and was bearing down with most of his weight the entire time Mr Floyd lay face down with his hands cuffed behind his back, a use-of-force expert said at Chauvin’s murder trial.

Jody Stiger, a Los Angeles Police Department sergeant serving as a prosecution witness, said that based on his review of video evidence, Chauvin’s knee was on Mr Floyd’s neck from the time officers put Mr Floyd on the ground until paramedics arrived, about 9 1/2 minutes, by prosecutors’ reckoning.

Prosecutor Steve Schleicher showed jurors a composite image of five photos taken from various videos of the arrest.

Mr Stiger went through each photo, saying it appeared that the Minneapolis officer’s left knee was on Mr Floyd’s neck or neck area in each one.

“That particular force did not change during the entire restraint period?” Mr Schleicher asked.

“Correct,” Mr Stiger replied.

His evidence came a day after Chauvin lawyer Eric Nelson sought to point out moments in the video footage when, he said, Chauvin’s knee did not appear to be on Mr Floyd’s neck.

Mr Nelson also has suggested that bystanders who were yelling at Chauvin to get off Mr Floyd distracted the officers.

On Tuesday, the defence lawyer got some police witnesses to acknowledge that jeering onlookers can make it more difficult for officers to do their duty.

But Mr Stiger told Mr Schleicher: “I did not perceive them as being a threat”, even though some onlookers were name-calling and using foul language.

He added that most of the yelling was due to “their concern for Mr Floyd”.

It was Mr Stiger’s second day in the witness box.

On Tuesday, he said officers were justified in using force while Mr Floyd was resisting their efforts to put him in a squad car.

But once Mr Floyd was on the ground and stopped resisting, officers “should have slowed down or stopped their force as well”.

Mr Stiger said that after reviewing video of the arrest, “my opinion was that the force was excessive”.

Several experienced officers, including the police chief himself, have given evidence as part of an effort by prosecutors to dismantle the argument that Chauvin was doing what he was trained to do when restrained Mr Floyd last May.

According to evidence and records submitted on Tuesday, Chauvin took a 40-hour course in 2016 on how to recognise people in crisis, including those suffering mental problems or the effects of drug use, and how to use de-escalation techniques to calm them down.

Sergeant Ker Yang, the Minneapolis police official in charge of crisis-intervention training, said officers are taught to “slow things down and re-evaluate and reassess”.

Records show Chauvin also underwent training in the use of force in 2018.

Lieutenant Johnny Mercil, a Minneapolis police use-of-force train, gave evidence that those who attended were taught that the sanctity of life is a cornerstone of departmental policy and that officers must use the least amount of force required to get a suspect to comply.

Under cross-examination by Mr Nelson, Mr Mercil said officers are trained in some situations to use their knee across a suspect’s back or shoulder and employ their body weight to maintain control.

But Mr Mercil added: “We tell officers to stay away from the neck when possible.”

Mr Nelson has argued that the now-fired white officer “did exactly what he had been trained to do over his 19-year career”, and he has suggested that the illegal drugs in Mr Floyd’s system and his underlying health conditions are what killed him, not Chauvin’s knee.

Mr Nelson showed Mr Mercil several images taken from officers’ body-camera videos, asking after each one whether it showed Chauvin’s knee appearing to rest more on Mr Floyd’s back, shoulder or shoulder blades than directly on Mr Floyd’s neck.

Mr Mercil often agreed.

Mr Nelson acknowledged the images were difficult to make out.

They were taken at different moments during Mr Floyd’s arrest, starting about four minutes after he was first pinned to the ground, according to time stamps on the images.

Chauvin, 45, is charged with murder and manslaughter in Mr Floyd’s death on May 25.

Mr Floyd, who was 46 and a black man, was arrested outside a neighbourhood market after being accused of trying to pass a counterfeit 20 dollar note.

A panicky-sounding Mr Floyd writhed and claimed to be claustrophobic as police tried to put him in the police car.

Bystander video of Mr Floyd crying that he could not breathe as onlookers yelled at Chauvin sparked protests around the US that descended into violence in some cases.

Instead of closing ranks to protect a fellow officer behind what has been dubbed the “blue wall of silence”, some of the most experienced members of the Minneapolis force have openly condemned Chauvin’s actions as excessive.