(AP) -- A crew of prison guards known as the "Beat Up Squad" repeatedly punched and kicked a handcuffed inmate with mental problems and threw him down a staircase before he died, his family alleged in a lawsuit filed Wednesday.

The suit in federal court in Manhattan accuses prison guards of trying to cover up the fatal beating of Samuel Harrell at Fishkill Correctional Facility by claiming that he had assaulted them before dying of a synthetic marijuana overdose.

An autopsy found that Harrell had no illicit drugs in his system and that he died of cardiac arrhythmia "following a physical altercation with corrections officers." The medical examiner ruled it a homicide.

"We're all here to seek justice," the victim's father, Samuel Harrell Sr., of Houston, said at a news conference announcing the lawsuit. "This was a criminal act ... that took the life of my son."

The suit claims that the death was part of a pattern of brutality by rogue guards at the prison, located about 70 miles north of New York City. It says the misconduct was tolerated by supervisors and officials and condoned by union representatives.

"But for their inaction, it's our belief that Samuel Harrell would have been alive today," said attorney Jonathan Moore, who's representing the family.

The state Department of Corrections had no comment on the lawsuit, which was seeking unspecified damages.

Harrell, 30, was serving an eight-year term for a drug conviction at the time of his death and had a history of erratic behavior stemming from bipolar disorder.

On April 21, Harrell began acting delusional, packing his bags and telling people he was going home, the suit says. According to witnesses, Harrell, who was black, was eventually surrounded by a dozen or more white guards who tackled and handcuffed him while using racial slurs. Then, witnesses said, the guards beat and stomped him until he was unconscious.

In one of several sworn statements supporting the suit, one inmate claimed that he heard an officer say, "Throw him down the stairs." Another recalled seeing Harrell "on the floor, bent in an impossible position. His eyes were open but they weren't looking at anything."

The second inmate also claimed that a guard grabbed him by the neck and warned, "You better forget what you saw here if you ever want to make it home."

Last month, federal prosecutors announced they were joining an investigation by local authorities into Harrell's death.

No criminal charges have been filed in the case.