One of the extraordinary aspects of Mr. Chauvin’s state trial was the number of officers who took the witness stand to disavow the actions of their former colleague. “To continue to apply that level of force to a person proned out, handcuffed behind their back — that in no way, shape or form is anything that is by policy,” said the Minneapolis police chief at the time, Medaria Arradondo, from the witness stand.

And outside the courtroom, law enforcement officials around the country cheered the convictions.

This time could be different. While prosecutors are expected to call officers to the stand to testify about the training the officers had on what to do when they see another officer using excessive force, the wider law enforcement community may have more sympathy for the three officers than they had for Mr. Chauvin, said lawyers who have been involved in legal cases against officers.

“Every cop out there is going to see themselves in their position,” said John Marti, a former federal prosecutor in Minnesota. “They are going to remember back to the day when they were a young officer and they had the old bulls running around probably using heavy-handed force. And they are going to remember that day and think to themselves, ‘There’s no way I would have stood up to my training officer.’”

Mr. Chauvin, for his part, will loom over the proceedings. When nearly 300 prospective jurors arrived at the federal courthouse here Thursday morning, one of the first things the judge told them is to disregard anything they heard about Mr. Chauvin’s crimes.

“The crimes that Mr. Chauvin pleaded guilty to are totally separate to those at issue here,” said Judge Paul A. Magnuson, who was appointed to the federal bench by President Reagan in 1981.

A jury of 18 people — 12 regulars and six alternates — was selected in one day last week, a sharp contrast to the nearly two weeks it took to pick a jury in Mr. Chauvin’s state murder trial. The panel in this case is overwhelmingly white, partly a reflection that in federal cases courts can call jurors from across the state, including from areas that are whiter and more conservative than the Minneapolis metro area.

Experts say it is only a remote possibility that Mr. Chauvin is called to testify, although in his plea agreement he acknowledged that he was “aided and abetted” by his fellow officers, and that he had never threatened them to “disregard or fail to comply” with department policies.

“You don’t want to have John Gotti cooperating against his soldiers,” Mr. Marti said. “You want his soldiers cooperating against John Gotti. You want to cooperate up, you don’t want to cooperate down.”