After spending nearly a quarter-century behind bars, John McMillon was counting down to the day that he would be eligible to be released on parole.

“I’m going home,” a fellow inmate recalled him saying.

Mr. McMillon suffered from mental illness and had had disciplinary problems over the years. He was punished for assaulting a guard and for fighting with another inmate, but had then vowed to stay out of trouble, and was looking forward to his parole board hearing.

He never made it there.

One day in January, Mr. McMillon, 67, was involved in a struggle with guards at the Great Meadow Correctional Facility in upstate New York. According to the official account, he once again attacked a guard and resisted efforts to control him. He died of a heart attack, the official autopsy said.