CHICAGO (AP) — A federal trial against three former Minneapolis police officers seeks to hold them responsible for not stopping George Floyd’s murder — and perhaps strike a blow against long-standing police culture that breeds reluctance to rein in fellow officers.

The circumstances of Floyd’s death — pinned under Officer Derek Chauvin’s knee for more than nine minutes, recorded from multiple camera angles — may help prosecutors clear the bar for conviction on a charge that’s rarely brought, in part because it can be difficult to prove. Regardless, several former federal prosecutors and legal experts see a message in the Department of Justice’s pursuit of charges accusing J. Alexander Kueng, Thomas Lane and Tou Thao of violating Floyd’s civil rights.

“This sends a real message to counterbalance that very strong cultural set of influences in policing that often prevent an officer from stepping forward and reporting or stopping misconduct,” said Jonathan Smith, former chief of the department division that oversees police civil rights inquiries.

The federal charges require prosecutors to prove the ex-officers willfully deprived Floyd of his constitutional rights — meaning that they knew what they were doing was wrong and still went ahead.

Chauvin was convicted of state murder and manslaughter charges last year and pleaded guilty to a federal civil rights charge. Kueng, Lane and Thao also face a separate state trial on charges they aided and abetted murder and manslaughter.

Police departments and local prosecutors have their own means to punish officers who don’t intervene. But high-profile examples show how risky it is, both personally and professionally, for officers who do intervene or who cooperate with investigations of fellow officers.

In Chicago, a key police witness against three Chicago officers charged with trying to cover up the 2014 shooting of Laquan McDonald testified that she was taunted as a “rat” by fellow officers who said her calls for assistance while on duty should be ignored.

In Florida late last year, an officer with less than three years experience pulled a sergeant by his belt away from a handcuffed suspect, apparently afraid he was about to pepper spray the man. The sergeant, a 21-year veteran, grabbed the officer — at one point placing his hand against her throat.

And in Buffalo, New York, Officer Cariol Horne was fired in 2008 after an arbitration process ruled that she had put other officers at risk when she stopped a fellow officer whose arm was around a handcuffed suspect’s neck.

The Florida officer drew the support of her chief for stepping in, though his public comments came only after a Miami TV station published video of the conflict. Horne, the Buffalo officer, was ultimately granted a pension — but only after a lengthy court battle and a push to change state law.

Floyd’s May 2020 killing prompted many police leaders to boost their own training on officers’ duty to intervene when a fellow officer puts someone in danger.

Since mid-2020, 21 of the country’s 100 largest police departments adopted policies on officers’ duty to intervene and lawmakers in 12 states have approved similar laws, according to the Council on Criminal Justice’s Task Force on Policing.

Minneapolis added a “duty to intervene” policy in 2016. Days after Floyd’s murder, city officials agreed to strengthen it. The state Human Rights Department now can take the Minneapolis Police Department to court for any violations.

Joseph Giacalone, a professor at John Jay College of Criminal Justice and a retired New York police sergeant, said officers know the consequences of federal prosecution far outweigh those of internal sanctions.

“Police know there’s no limit to what the federal government can do,” Giacalone said. “I definitely think now the risk of a federal charge is in the back of the mind.”

Federal civil rights violations that result in death are punishable by up to life in prison or even death, but those sentences are extremely rare. Federal sentencing guidelines rely on complicated formulas that indicate the officers in Floyd’s killing would get much less if convicted.

During opening statements to jurors in the Floyd case, Kueng’s defense attorney noted that Chauvin was the most senior officer present and called “ all of the shots.” Later in the week, the attorney sought to show the department instilled a sense of obedience in recruits and failed to show effective ways of intervening when force is misused.

For some experts, the case’s spotlight on Kueng, Lane and Thao represents yet another opportunity to jolt individual police officers and department leaders into action and potentially prevent dangerous misconduct that erodes trust in the entire profession.

“We can’t stop and think it was just Chauvin,” said Kami Chavis, a professor at Wake Forest University School of Law. “Typically, within a police department, it’s a small percentage of officers that are going to behave in that way. But when that small group is allowed to go unchecked, you are telegraphing to others what the culture is and that this is tolerated.”