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When officer Derek Chauvin pressed his knee to George Floyd's neck Monday night, he used a restraint once taught by the department that is no longer sanctioned by most Minnesota law enforcement agencies.

Floyd died after complaining that he couldn't breathe while being restrained.

The maneuver, billed as a means to gain control of a thrashing suspect, requires pressure on the side of an individual's neck.

At Hennepin Technical College, which trains about half of Minnesota's police officers, students were taught to use a form of the technique until at least 2016, said Mylan Masson, a longtime Minneapolis police officer and former director of the college's law enforcement and criminal justice education center.

"Once the [officer] is in control, then you release," Masson said. "That's what use of force is: You use it till the threat has stopped."

Floyd's death drew strong condemnation from several law enforcement experts after a cellphone video of the arrest began circulating on the internet.

"It was outrageous, excessive, unreasonable force under the circumstances," said George Kirkham, a professor emeritus at the College of Criminology and Criminal Justice at Florida State University.

"We're dealing with a [suspected] property offender," added Kirkham, who has written extensive police training materials on excessive force. "The man was prone on the ground. He was no threat to anyone."

The Minnesota Chiefs of Police Association commended Chief Medaria Arradondo on his "swift action" in firing the four officers.

"The actions of the former officers depicted in the video are in stark contrast to the values the chief has worked to instill," the organization said in a statement.

The video of Floyd's arrest sparked criticism, with veteran law officers questioning why the arresting officer did not lift his knee off Floyd's neck, despite his pleas and those of bystanders.

Masson said that while she doesn't know all the facts of the Floyd case, she was troubled by the prolonged use of the maneuver on a handcuffed man who kept saying he couldn't breathe.

Former Hennepin County Sheriff Rich Stanek said the video "definitely shocks the conscience. It appears the officers were on autopilot. They ignored the surroundings."

"The victim is saying, 'I cannot breathe,' " Stanek said. "When he is no longer verbalizing, you got to pay attention."

Norm Stamper, the former chief of the Seattle Police Department, also expressed dismay.

"If this officer were facing a life-threatening situation, I would still be questioning his techniques," Stamper said. He noted that the video doesn't show what led to the point where the officer is pressing down on Floyd's neck.

Attorney Robert Bennett has represented the family of David Smith, who was killed when officers placed a knee on his back and held him down when he was acting disruptive at the downtown Minneapolis YMCA in 2010.

Unable to breathe, Smith died, and the medical examiner's office ruled it a homicide by police. The city settled the suit Bennett filed for $3 million.

"It certainly proves the Minneapolis police haven't learned anything since the death of David Smith," said Bennett.

Kirkham, the criminologist and a former police officer, said that police officers in the United States are trained that the head and neck are a "red zone" — areas to be avoided.

If an officer's life is threatened or they face great bodily harm they can strike someone with a baton, he said.

But he said that putting a knee on a neck is not a police academy technique because of the risk. Arteries run along each side of the neck. Compression can cut off blood to the brain, causing brain death in a few minutes, he said.

Also Kirkham said it was wrong for another officer to stand by silently while Floyd was pinned to the ground in such a way.

"If you see an officer endangering the life of a person, you have a constitutional duty to step [in] and stop that, and your failure to do so is a both a crime and a tort — you can be sued," Kirkham said.

John G. Peters of Henderson, Nev., is president of the Institute for the Prevention of In-Custody Deaths, an organization that trains police officers.

"As police practice, officers are taught not to put their knee or a shin on a person's neck, particularly the side of the neck," Peters said.

The practice was acceptable 30 years ago, he said, "but not today, and it hasn't been for some time."