Infamous Boston gangster James "Whitey" Bulger who was convicted in 2013 of 11 murders and had inspired several films, has been found dead in a US federal prison in West Virginia.
The 89-year-old was discovered unresponsive at the maximum security facility shortly after being transferred from a Florida jail. The BBC quoted a prison official saying it was being probed as a homicide.
Bulger was captured in California in 2011 after a 16-year manhunt and sentenced to life in prison.
His death occurred after he was transferred to the Hazelton penitentiary that houses 1,385 inmates, according to reports.
The Federal Bureau of Investigation is handling the probe into Bulger's death, which occurred a day after his transfer to the West Virginia facility, according to the Federal Bureau of Prisons.
Bulger was found unresponsive at 8.20 a.m. on Tuesday, according to a statement from the prisons bureau.
He was pronounced dead by the Preston County medical examiner after failed lifesaving measures. No staff or other inmates were injured, it added.
An inmate with mafia ties was under investigation for the killing, three sources briefed on the incident told the Boston Globe.
The slain mob boss was severely beaten up by one or more of his fellow inmates only hours after his arrival, law enforcement sources told CBS.
Boston-based WFXT-TV, a Fox affiliate, reported that Bulger was killed after being admitted to the general inmate population.
"He lived violently and he apparently died violently," CNN quoted Dick Lehr, author of "Whitey: The Life of America's Most Notorious Mob Boss" as saying.
"It marks the full circle of a terrible life," Lehr said.
"Hopefully the seven years he spent in prison, as well as his recent death, brings some closure to the families of his many victims," Brian Kelly, one of the former federal prosecutors who tried Bulger, said in a statement.
The former leader of South Boston's Winter Hill gang inspired the film "Black Mass" featuring Johnny Depp, and "The Departed" , which won the Academy Award for Best Picture in 2007.
(With inputs from agencies.)