On the night of Samuel Harrell’s death in April, Lucas Renfrow, an inmate at Fishkill Correctional Facility, said he was walking up a cellblock stairwell when Mr. Harrell’s body came rolling down toward him. Shortly after, Mr. Renfrow recalled being shoved into a wall by a corrections officer and warned to keep quiet about what he had seen.

“The rest of my time in jail was hell,” said Mr. Renfrow, 30, who was released from the medium-security prison in Beacon, N.Y., on Aug. 27.

He said he was repeatedly harassed by guards, placed into solitary confinement on false charges and had a homemade weapon planted in his bunk. At one point, he said, he was threatened by corrections officers in the mess hall. “You’re going to be by yourself, and you’re going to belong to us,” he recalled them saying.

On Wednesday, lawyers for Mr. Harrell’s family filed a lawsuit in federal court claiming that more than a dozen officers were responsible for his death, which a medical examiner ruled a homicide. The lawsuit further claims that several inmates, including Mr. Renfrow, were “physically and verbally threatened” by guards after witnessing the death. Nearly all the inmate witnesses, some of them classified as severely mentally ill, were “placed in solitary confinement without cause,” said the complaint, which was filed by the law firm Beldock Levine & Hoffman.