The family of deceased Illinois inmate Larry Earvin is demanding to know the timeline and circumstances leading to his death in June.
Their attorney wants the Illinois Department of Corrections (IDOC) to stop "stonewalling" the investigation.
Earvin's son, niece and their attorney held a news conference Wednesday in Chicago.
"I was upset," said Regina Earvin, the inmate's niece. "I was close to him, and to know that he had been beat up by prison guards. It hurt me."
An IDOC spokeswoman said the agency referred the May 17 "incident" at the prison to the Illinois State Police, which turned the case over to the FBI. She said IDOC has "cooperated fully with authorities."
The FBI confirmed Wednesday they are investigating the incident.
Earvin, 65, was in custody at the Western Illinois Correctional Center in May when there was an "altercation with correctional staff," and his eventual death was a ruled a homicide, according to an autopsy report obtained by The Associated Press.
"Mr. Earvin was brutally and viciously beaten to death by four cruel and savage employees of the Illinois Department of Corrections," said Michael Oppenheimer, the Earvin family's attorney. "This was a sustained, extensive ruthless murder."
The autopsy report, according to the attorney's office, states that Earvin died from "blunt abdominal trauma" in June, and cited the following injuries: 15 rib fractures, dozens of abrasions and puncture wounds.
The report also states that part of Earvin's colon had been surgically removed and an ileostomy bag used to collect waste, according to the attorney's news release.
The alleged incident took place on May 17 at the prison in Mount Sterling, and Earvin died six weeks later, the AP reported. Earvin was serving a six-year sentence for a Cook County robbery and was scheduled for release this past September.
Four Western Illinois employees were placed on administrative leave with pay on May 22, according to documents obtained by the AP. Although the employees' names have been reported, ABC7 will not release their identities because no charges have been filed against them.
An IDOC spokeswoman said the employees have been "administratively relieved of duty."
The family said Earvin had a history of schizophrenia and was also bipolar.
"He has a record of mental illness. To be locked up for something under $300 for 6 years, it just wasn't right. He didn't deserve to be in there in the first place. He needed help," said Larry Pippion, the inmate's son.
The attorney added that, even with mental illness, IDOC is responsible for treating inmates.
"They are responsible for making sure he has his meds. And when he doesn't take his meds, they are responsible for helping him. These people were supposed to care for him and not murder him," Oppenheimer said.
On Wednesday, Clinton County Coroner Phillip Moss declined to release the autopsy report and said he was "advised by the FBI not to release any of that information. You have to contact the FBI or Illinois State Police."
Illinois State Police Sgt. Jacqueline Cepeda said in an email: "Illinois State Police assisted IDOC with this investigation, which is now being handled by the FBI. Please reach out to the FBI for further information regarding this case."
Family of inmate Larry Earvin, who died in Illinois prison, demands answers