A police officer's duty to intervene when a colleague uses excessive force is at the core of the prosecution of three men charged as Derek Chauvin's accomplices in the murder of George Floyd.

Former Minneapolis police officers J. Alexander Kueng, Thomas Lane and Tou Thao are being tried in federal court in St. Paul on charges they violated Floyd's civil rights by failing to provide him with medical aid as Chauvin knelt on his neck for more than nine minutes and eventually killed him. Kueng and Thao are also charged with failing to intervene on Floyd's behalf.

Kueng and Lane were new members of the force at the time. Chauvin, a 19-year police veteran, was at one time Kueng's field training officer.

Floyd died after being detained by Chauvin and his fellow officer on the pavement at E. 38th Street and Chicago Avenue on May 25, 2020.

Chauvin was convicted in Hennepin County District Court last year and is serving a 22½-year state sentence for murder. He pleaded guilty to federal charges in December.

Kueng, Lane and Thao are scheduled to be tried in state court on charges of aiding and abetting murder.

Prosecutors argue that the Police Department's regulation requiring officers to intervene in cases of excessive force was part of their initial training. The say it is "a foundational principal of policing that permeates many aspects of training and practice," according to a pre-trial court filing.

Defense attorneys counter that training on intervention was minimal, limited to a reference in a presentation, with no scenario-based instruction that would give officers the necessary tools to prevent a superior's misbehavior.

Unlike the Chauvin trial, which focused on his actions, this time the Minneapolis Police Department's culture and practices are under the microscope.

"This is a bold prosecution and a meritorious one," said former U.S. Attorney Tom Heffelfinger. But he said it is also a challenging case for the U.S. attorney's office.

"Unlike most prosecutions where the allegation is about what somebody did, here the allegation is based on what they didn't do," Heffelfinger said.

In the aftermath of Floyd's murder, Minneapolis police officials, like many of their counterparts across the country, have come to recognize that intervention training must be upgraded.

Officers in Minneapolis, using training modules developed at the Georgetown Law School under a program called ABLE, began scenario-based training late last year on how to stop a fellow officer from engaging in misconduct. ABLE stands for Active Bystandership for Law Enforcement. It stresses that even rookie officers must be prepared to intervene when senior officers are in the wrong and that senior officers, including the top command staff, must embrace the concept.

More than 200 departments nationwide, including the U.S. Capitol police, have signed up for ABLE training It is an outgrowth of a peer intervention training program developed in New Orleans eight years ago called EPIC, short for Ethical Policing Is Courageous.

"It's a fascinating case and an important one," said Mark Osler, a former federal prosecutor and now a law professor at the University of St. Thomas. "It is presenting the question of police culture, the culture to stand aside when someone is doing something obviously wrong."

It is rare to charge officers with failure to intervene. However, over the past decades, federal courts occasionally have affirmed an officer's obligation to stop a colleague from assaulting a suspect.

"We believe it is clear that one given the badge of authority of a police officer may not ignore the duty imposed by his office and fail to stop other officers who summarily punish a third person in his presence or otherwise within his knowledge," the U.S. Seventh Circuit Court of Appeals said in 1972.

In 1981, the Eighth Circuit Court of Appeals ruled that a subordinate can be held jointly liable "for failing to intervene if a fellow officer, albeit his superior was using excessive force and otherwise was unlawfully punishing the prisoner."

Attorneys for Kueng and Lane have argued that neither officer recalled training at the department's police academy that putting a knee on a suspect's neck was acceptable procedure, but that they didn't intervene because they figured it was reasonable, as Chauvin was a longtime department veteran.

Retired Minneapolis police officer Scott Dahlquist said in an interview that he isn't surprised that the junior officers would have deferred to a veteran like Chauvin. In high-stress situations, he said "you cannot rule by committee; no, somebody has to be in charge."

He said that because Chauvin didn't carry out any obvious forms of violence — "shots, blows or kicks" — the other officers may not have immediately recognized that his actions had gone too far.

John Marti, former acting U.S. attorney in Minneapolis, said this is a more challenging case for the government because of the hierarchal nature of the Police Department, not unlike that of the Marine Corps where he spent over 20 years, including 12½ years on active duty.

"If somebody lower in the chain of command was to willfully disobey the command, they better be right because they could be disciplined for violating orders," said Marti, who is now in private practice at Dorsey & Whitney law firm.

The government must prove that the officers acted willfully, and much will depend on how the judge instructs the jury, he said. "It is likely to be that the officers knew they had an obligation to act and fully chose not to," Marti said.

"If I am a prosecutor, I'm going to say what Derek Chauvin was doing was clearly unlawful and you don't need 100 hours training to know he was murdering George Floyd. You don't need graduate courses. It was plain what was happening at that time."

That point is underscored by Dave Bicking of Communities United Against Police Brutality (CUAPB), a local activist group. Bicking said the three officers should have known better.

"The training was clearly inadequate, however, that does not mean it is a defense against the federal charges," he said. "Remember, they were close to bystanders there, and none of [the bystanders] had received training and intervention, yet all of them recognized that intervention was needed. You don't need a full EPIC training to be a caring human being."

Bicking's group pressed Minneapolis Police Chief Medaria Arradondo to adopt an EPIC-style program in 2017, but Arradondo decided not to do so until last summer. In the meantime, St. Paul Police Chief Todd Axtell began implementing such a program for his department before Floyd's death.

Ervin Staub, an emeritus professor of psychology at the University of Massachusetts Amherst, is a recognized expert on why individuals don't speak up when they see an injustice. He also is considered the guru that has led to the current surge in training programs through ABLE that teach officers to intervene.

"If those officers (Keung, Thao and Lane) had ABLE training, it is highly probable they would have intervened," Staub said. But just because they did not get the ABLE training does not free them from responsibility, he said.

"The law assumes that officers have an obligation to intervene and tragically they did not," Staub said.

Genevieve Hansen, an off-duty Minneapolis firefighter, testified Wednesday that she happened on the scene as Chauvin pinned Floyd to the pavement. She said she offered to render medical aid but was rebuffed by Thao.

"Her instinct was to try to save George Floyd, and the contrast between her and the four officers is pretty stark," Osler said.

During the fatal encounter, Lane suggested to Chauvin that he turn Floyd over but was ignored. That likely was a factor in the federal grand jury's decision to not indict him on the first count of failing to intervene to stop Chauvin. Instead, he was indicted on the second count of failure to provide medical aid that could have kept Floyd alive. All four officers were indicted on that count.

Lane, under instructions from a paramedic, gave Floyd CPR in an ambulance, although it was four minutes after Floyd no longer had a pulse.

"The weakest charge is against former officer Lane," Osler said. "The biggest question possibly in the jury's mind is should he be held liable. He simply has different facts than the other two."