(CNN) - A Los Angeles Police Department use-of-force expert testified Wednesday that former police officer Derek Chauvin used "deadly force" by holding his knee on the neck of George Floyd for more than 9 minutes in a situation where no force was necessary.
LAPD Sgt. Jody Stiger, in his second day on the stand, said the pressure of Chauvin's body weight on the back of Floyd's neck could have caused potentially lethal "positional asphyxia."
"He was in the prone position. He was not resisting. He was handcuffed. He was not attempting to evade. He was not attempting to resist," Stiger said of Floyd. "And the pressure ... that was being caused by the body weight could cause positional asphyxia which could cause death."
Stiger testified the dangers of positional asphyxia have been known in law enforcement for at least 20 years.
He told the jury that "no force should have been used," with three officers restraining Floyd and two others standing by.
Chauvin is also seen in the body camera video grabbing Floyd's fingers in an attempt to inflict pain to get him to comply, Stiger told the court. He was asked what if Floyd couldn't comply.
"At that point it is just pain," Stiger responded.
Stiger took the stand as the trial entered its eighth day of testimony Wednesday, with prosecutors taking aim at Chauvin's actions on May 25, 2020, with a series of policing experts testifying about proper training.
This week's testimony has countered the defense's argument that Chauvin "did exactly what he had been trained to do" when he restrained Floyd. Prosecutors have sought to show he used excessive and unreasonable force on Floyd and had a "depraved mind" without regard for human life.
The focus on police policy and training comes after a first week of testimony centered on what happened to Floyd on his last day. The evidence included cellphones, surveillance cameras and police body cameras; testimony from distressed bystanders; descriptions from paramedics and police supervisors who responded to the scene; and Chauvin's own statements about what happened.
Chauvin, 45, has pleaded not guilty to second-degree murder, third-degree murder and third-degree manslaughter. Nelson has not indicated whether Chauvin will testify in his own defense.
On Tuesday, a Minneapolis Police use-of-force training instructor said Chauvin's kneeling on George Floyd's neck is not a trained neck restraint tactic.
A crisis intervention training coordinator and a police CPR instructor each told the jury that officers are required to de-escalate situations and to render aid to those in distress.
The trial, now in its second full week of testimony, is expected to last about a month.
LAPD Sgt. says bystanders were not a threat
Sgt. Stiger testified that the crowd of bystanders gathered at the scene did not pose a threat to Chauvin or other officers -- an assertion made by the defense, which described the crowd as hostile.
"They were merely filming, and most of their concern was for Mr. Floyd," the expert testified.
While it is possible for a crowd to distract an officer, Stiger does not believe it happened in this case because Chauvin was talking to Floyd.
"In the body-worn video, you can hear Mr. Floyd displaying his discomfort and pain, and you can also hear the defendant responding to him," he said.
On cross-examination, he said some of the comments could be considered potential threats and that officers are taught to predict future behavior. He also acknowledged that Chauvin could have used a Taser initially because Floyd actively resisted attempts to put him in a police vehicle.
At one point, Nelson played a video of Floyd, handcuffed on the ground, and asked Stiger if Floyd could be heard saying, "I ate too many drugs." Stiger said he could not make out what Floyd was saying.
Stiger's testimony initially began on Tuesday afternoon. He said that he has conducted over 2,500 use-of-force reviews.
The sergeant said officers were initially justified in using force when Floyd actively resisted arrest and refused to get into the squad car. Floyd also kicked at officers when he was first taken to the ground, body camera video shows. The circumstances then changed.
"However, once he was placed in a prone position on the ground, he slowly ceased resistance and at that point the ex-officers, they should have slowed down or stopped their force as well," Stiger said.
He said his opinion was based on the standard of what an "objectively reasonable" officer would do. That took into account the low-level seriousness of Floyd's underlying crime -- allegedly using a $20 counterfeit bill -- as well as his actions, MPD policies and what officers knew at the time.
"They should have de-escalated the situation, or attempted to," Stiger said. Instead, "they continued the force that they were utilizing from the time that they first put him on the ground."
Minneapolis Police Lt. says they don't teach leg-neck restraints
While neck restraints may be allowed on suspects actively resisting, they are not to be done with the knee and they would not be authorized on a suspect who is handcuffed and under control, testified Lt. Johnny Mercil, the Minneapolis Police use-of-force training instructor.
"We don't train leg-neck restraints with officers in service, and as far as I know, we never have," Mercil said Tuesday.
Officers are taught to only use force that is proportional to the threat.
"You want to use the least amount of force necessary to meet your goals," Mercil said. "If you can use a lower level of force to meet your objectives, it's safer and better for everyone involved."
He also testified that handcuffed suspects can have difficulty breathing on their stomachs. He said officers are trained to move suspects into a side recovery position -- "the sooner the better."
However, Mercil said in cross-examination that Chauvin's position might be considered "using body weight to control," a tactic in which officers place a knee on a prone suspect's shoulder blades to handcuff them. He acknowledged that some screen grabs of police body-camera footage show Chauvin with his knee on Floyd's shoulders.
"However, I will add that we tell officers to stay away from the neck when possible, and if you're going to use body weight to pin, to put it on their shoulder and be mindful of position," he said.
Mercil said that the position is transitory and is meant to end once the suspect is under control.
CNN's Eric Levenson contributed to this report.