A use-of-force expert testified Monday in federal court that the conduct of three former Minneapolis police officers charged with violating George Floyd’s civil rights was "inconsistent" with generally accepted policing practices, but a defense attorney questioned the quality of his analysis.

Tim Longo, the police chief at the University of Virginia at Charlottesville, testified in a St. Paul, Minnesota, courthouse in the trial against J. Alexander Kueng, Thomas Lane and Tou Thao and said he reviewed videos, Minneapolis Police Department policies, training materials and other items to evaluate the conduct.

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The officers are charged with violating Floyd's constitutional rights while acting under government authority. All three are accused of depriving Floyd, a 46-year-old Black man, of medical care while he was handcuffed, facedown as Officer Derek Chauvin pressed his knee onto Floyd’s neck for 9 1/2 minutes. Kueng knelt on Floyd’s back and Lane held down his legs while Thao kept bystanders back.

This combination of photos provided by the Hennepin County Sheriff's Office in Minnesota on  June 3, 2020, shows, from left, former Minneapolis police officers J. Alexander Kueng, Thomas Lane and Tou Thao. 

This combination of photos provided by the Hennepin County Sheriff's Office in Minnesota on  June 3, 2020, shows, from left, former Minneapolis police officers J. Alexander Kueng, Thomas Lane and Tou Thao.  (Hennepin County Sheriff's Office via AP, File)

Kueng and Thao are also accused of failing to intervene to stop the May 25, 2020, killing, which triggered protests worldwide and a reexamination of racism and policing.

The charges allege that the officers’ actions resulted in Floyd’s death.

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Lane, who is White; Kueng, who is Black; and Thao, who is Hmong American, also face a separate state trial in June on charges alleging that they aided and abetted murder and manslaughter.

Longo testified that officers are allowed to use only as much force as is proportionate to the level of resistance they are encountering, and they are required to stop using force when resistance stops. They are also required to try to de-escalate situations. Longo said he found no reason to put Floyd on the ground, and that once Floyd was there, he wasn't resisting and was not a threat.

In this image from police body camera video shown as evidence in court, paramedics arrive as Minneapolis police officers, including Derick Chauvin, second from left, and J. Alexander Kueng restrain George Floyd in Minneapolis, on May 25, 2020.

In this image from police body camera video shown as evidence in court, paramedics arrive as Minneapolis police officers, including Derick Chauvin, second from left, and J. Alexander Kueng restrain George Floyd in Minneapolis, on May 25, 2020. (Minneapolis Police Department via AP, File)

Longo said an officer's "duty of care" for people in custody "is absolute" because they are restrained and cannot care for themselves. He also said an officer has a duty to take "affirmative steps" to stop another officer from using excessive force.

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"The term intervene is a verb, it’s an action word. And it requires an act. And what you do is, you stop the behavior," he said.

Longo testified that when it comes to a duty to provide medical aid, the actions of Thao and Kueng were inconsistent with generally accepted policing practices. He said Thao "didn't do anything" and Kueng took no further action after checking Floyd's pulse, and finding none.

Longo said that when Lane gave chest compressions to Floyd in an ambulance, he was fulfilling his duty to provide medical aid. But, he said, that does not apply to what happened before that point, when no aid was given.

In this image from surveillance video, Minneapolis police Officers from left, Tou Thao, Derek Chauvin, J. Alexander Kueng and Thomas Lane are seen attempting to take George Floyd into custody in Minneapolis, Minn on May 25, 2020.

In this image from surveillance video, Minneapolis police Officers from left, Tou Thao, Derek Chauvin, J. Alexander Kueng and Thomas Lane are seen attempting to take George Floyd into custody in Minneapolis, Minn on May 25, 2020. (Court TV via AP, Pool, File)

Thao's attorney, Thomas Plunkett, suggested that Longo reviewed only materials that prosecutors "cherry picked" for him and didn’t look further. He pressed a defense theme that the Minneapolis Police Department bears responsibility for Floyd's killing for training its officers poorly.

Plunkett played part of a training presentation that showed violent confrontations between officers and subjects, with audio of a locker room speech given by actor Al Pacino, playing a coach in the 1999 football movie "Any Given Sunday," exhorting his players to fight "inch by inch" because it will make the difference between winning and losing — and living and dying.

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Plunkett said the video is the last thing recruits see in their use-of-force training and asked if it was consistent with accepted police policies and practices. Longo replied that he found the video "very disturbing," because of its images of people dying and getting hurt, but that he didn't know the context in which it was presented.

Prosecutors began presenting their case on Jan. 24 and said late Friday that they expected to rest Monday. Defense attorneys will then start presenting witnesses. The attorney for Lane has said his client will testify. Attorneys for Thao and Kueng haven’t said if they will. 

The Associated Press contributed to this report.