The U.S. Bureau of Prison said Bulger "was fond unresponsive" at 8:20 a.m. and that emergency life-saving measures by prison staff were unsuccessful.
Prison officials haven't released details, but say his body was found sometime after he was moved to the prison.
Bulger arrived at the penitentiary on Monday after being processed at an Oklahoma City transfer site.
Federal Bureau of Investigation agents overseeing Bulger as an informant later testified that Bulger could commit any crime, except murder, in exchange for information. He was sentenced to serve two consecutive life sentences plus five years.
The Federal Bureau of Prisons online inmate log had listed Bulger as an inmate there early Tuesday morning, but by the afternoon he was no longer there.
Bulger, the model for Jack Nicholson's ruthless crime boss in the 2006 Martin Scorsese movie, "The Departed", led a largely Irish mob that ran loan-sharking, gambling and drug rackets.More news: 18 people, mostly schoolchildren, die in flash flood in Jordan
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Bulger, a Boston gangster convicted in 2013 of numerous crimes-including involvement in 11 murders, was recently transferred to USP Hazleton after being moved from a Florida prison and then an Oklahoma transfer facility.
A man who lived by violence, vengeance and intimidation, Bulger would not make eye contact at his sentencing with the relatives of the people he'd killed, nor those who were slain by his accomplices in the treacherous Winter Hill Gang.
Update: According to TMZ, Bulger's death is being treated as a homicide.
Bulger was born September 3, 1929, in Dorchester, Massachusetts.
"And my thing is, as far as I'm concerned, I didn't get a fair trial", Bulger said, when he revealed he would not testify.
Prosecutors said that Greig had numerous opportunities to leave Bulger or report him, but she never did.
Bulger was arrested with his girlfriend, Catherine Greig.