Notorious mob boss James "Whitey" Bulger was brutally beaten in the fatal attack Tuesday at the Hazelton prison in Bruceton Mills, according to law enforcement officials.
The South Boston mobster, 89, died a day after he was transferred there from another facility -- the victim of the kind of brutality he once handed out.
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At the time of the killing, Bulger was in the general prison population, which gave inmates easy access to him, according to a federal law enforcement official, who has knowledge of the investigation.
Bulger was found unresponsive at 8:20 a.m., and was pronounced dead after failed lifesaving measures, prison officials said.
Investigators believe he was attacked by more than one person. At least one of the inmates involved has ties to organized crime in Massachusetts, the official said.
The prison on Thursday suspended inmate visitation until further notice, according to a post on its website.
One suspect is a Mafia hit man
One of the two suspects in the brutal beating was identified as Fotios "Freddy" Geas, a Mafia hit man from Massachusetts, The New York Times reported, citing unnamed sources. It said Geas was moved to solitary confinement after the killing.
Geas, 51, is serving a life sentence at the same prison for the 2003 murder of a crime boss and another man he believed was an FBI informant.
The US Penitentiary Hazelton is a high-security facility that houses 1,270 male offenders.
No staff or inmates were injured in the attack, the prisons bureau said. Federal officials are investigating Bulger's death as a homicide.
'It marks the full circle'
A man who wrote a book about the much-feared crime boss said he died the way he once lived.
"He lived violently and he apparently died violently," said Dick Lehr, author of "Whitey: The Life of America's Most Notorious Mob Boss."
"It marks the full circle of a terrible life."
Bulger was arrested in June 2011 after he eluded federal authorities for more than 16 years. He was serving the rest of his life in prison for a series of crimes that included his role in 11 murders. Before he went on the run, he had been a longtime FBI informant.
A federal jury convicted him of 31 counts, including racketeering, extortion, money laundering, drug dealing and weapons possession. The jury found him culpable in 11 killings from 1973 through 1985.
He was sentenced in November 2013 to two life terms plus five years as architect of a criminal enterprise that a federal judge said committed "unfathomable" acts that terrorized a city.
The circumstances of this week's transfer remain unclear. Bulger had also been housed at federal penitentiaries in Oklahoma and Tucson, Arizona.
Captured nearly 2 decades later
Bulger skipped town ahead of a pending indictment and was captured in California a decade and a half later.
In 2012, his girlfriend, Catherine Greig, was sentenced to eight years in federal prison for identity fraud and helping the mob boss avoid capture. Before their arrest, they lived under false names in Santa Monica, in what Bulger described as a "16-year honeymoon."
After fleeing Massachusetts, investigators learned that the longtime mobster had been an FBI informant and his FBI handler had tipped him off to the charges.
His story has been the inspiration for several movies, including the 2006 Oscar-winning film "The Departed," which starred Jack Nicholson as a character modeled after him.
In 2015, actor Johnny Depp played him in the film "Black Mass."
Update: This article and headline have been updated to remove references to reporting that Bulger's attackers attempted to remove his tongue during the attack. CNN has not been able to independently confirm this information.