Oct. 31 (UPI) -- Federal authorities said Wednesday they're investigating the death of notorious Boston mob boss James "Whitey" Bulger as a homicide.
The 89-year-old was found dead Tuesday at USP Hazelton in Bruceton Mills, W.Va., within a day of his transfer to the facility. Officials did not reveal his cause of death.
The U.S. Attorney's Office for the Northern District of West Virginia released a statement saying it was working with the FBI to probe the homicide.
"To protect the integrity of the investigation, no further details will be released at this time," the release said.
Unnamed sources told the Boston Globe that a fellow inmate with ties to the mafia killed Bulger.
Officials found Bulger unresponsive around 8:20 a.m.
"Life-saving measures were initiated immediately by responding staff. Mr. Bulger was subsequently pronounced dead by the Preston County Medical Examiner," the U.S. Bureau of Prisons said in a statement.
A jury convicted Bulger in 2013 on 31 criminal counts for his involvement in 11 murders across the country during his time as leader of the Boston-based Winter Hill Gang.
A federal judge sentenced him to two life-in-prison terms plus five years in November 2013 and ordered him to pay $19.5 million in restitution and forfeit $25.2 million to the government.
"The testimony of human suffering that you and your associates inflicted on others was at times antagonizing to hear and painful to watch," Judge Denise Casper said during the sentencing. "The scope, the callousness, the depravity of your crimes, are almost unfathomable."
The FBI had been searching for Bulger for 16 years before his arrest. The bureau added him to its "10 Most Wanted Fugitives" list in 1999, offering $250,000 for information leading to his arrest. The manhunt even expanded to Britain after a businessman reported spotting him near London's Piccadilly Circus.
Prosecutors believe Bulger was responsible for more crimes during the 1970s, 80s and 90s, while he oversaw his criminal empire, than he was convicted of.