Infamous gangster James "Whitey" Bulger was "unrecognisable" after he was beaten to death in his cell by fellow inmates, reports say.
His body was discovered wrapped in a sheet after he failed to turn up for breakfast on Tuesday.
Reuters reported that the 89-year-old had been so badly beaten he was bleeding from his ears.
A prison employee, who has not been named, told the news agency that CCTV images show two men entered Bulger's cell less than 24 hours after he arrived at the jail in a wheelchair.
Once one of America's most wanted fugitives, Bulger was jailed in 2013 for being complicit in 11 murders.
His conviction followed 16 years on the run and his notoriety inspired several movies.
Jack Nicholson's character in The Departed was based on the career criminal, and he was played by Johnny Depp in the biographical film Black Mass.
The FBI has confirmed it is investigating the death, which a prison official said is being treated as murder.
Bulger's body was found so badly beaten that he was "unrecognisable", the New York Times reports.
Officials have not given a cause of death.
The Bureau of Prisons said no other inmates or staff were injured.
Bulger was reportedly moved to the Hazelton prison after spending only a week in an Oklahoma jail.
He had been moved there from a prison in Florida.
It is not known why Bulger had been forced to change jails so often recently.
JW Carney Jr, a lawyer who once represented the criminal, said in a statement that he "was sentenced to life in prison, but as a result of decisions by the Federal Bureau of Prisons, that sentence has been changed to the death penalty".
The US investigative journalist Michele McPhee said Paul Weadick, one of Bulger's underworld rivals, is an inmate at Hazelton prison.
Weadick was a co-defendant of Francis "Cadillac Frank" Salemme when he was convicted of the murder of a man in 1993.
Salemme headed up the La Costa organised crime family in Boston in the early 1990s.
Bulger was the city's other underworld boss at the time, and led the Winter Hill Gang that ran loan-sharking, gambling and drug rackets in the Boston area.
The mob boss lived a double life as an FBI informant.
His handler John Connolly Jr would look the other way with regards to the mob's crimes, with Bulger providing information about other gangsters in return.
The gangster became one of the FBI's "Top Ten Most Wanted Men" after he fled Boston in 1994.
Mr Connolly Jr had informed him that he was going to indicted.
Bulger spent 16 years as a fugitive before he was caught in Santa Monica, California, in 2011.
The gangster was feared for his short temper and brutality.
Prosecutors during his trial said he strangled two women with his hands and tortured a man for hours before shooting him in the head with a machine gun.
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Bulger was sentenced to two consecutive life terms plus five years for the 11 murders, plus other charges including racketeering.
He was was called "Whitey" because of his light blond hair but was said to detest the nickname and preferred being called Jimmy.