On the evening of April 21 in Building 21 at the Fishkill Correctional Facility, Samuel Harrell, an inmate with a history of erratic behavior linked to bipolar disorder, packed his bags and announced he was going home, though he still had several years left to serve on his drug sentence.

Not long after, he got into a confrontation with corrections officers, was thrown to the floor and was handcuffed. As many as 20 officers — including members of a group known around the prison as the Beat Up Squad — repeatedly kicked and punched Mr. Harrell, who is black, with some of them shouting racial slurs, according to more than a dozen inmate witnesses. “Like he was a trampoline, they were jumping on him,” said Edwin Pearson, an inmate who watched from a nearby bathroom.

Mr. Harrell was then thrown or dragged down a staircase, according to the inmates’ accounts. One inmate reported seeing him lying on the landing, “bent in an impossible position.”

“His eyes were open,” the inmate wrote, “but they weren’t looking at anything.”

Corrections officers called for an ambulance, but according to medical records, the officers mentioned nothing about a physical encounter. Rather, the records showed, they told the ambulance crew that Mr. Harrell probably had an overdose of K2, a synthetic marijuana.