According to the United States Department of Justice, Bulger was found unresponsive around 8:20 a.m. Officials attempted life-saving measures, but Bulger was subsequently pronounced dead.
Federal Bureau of Prisons officials and his attorney had declined to comment on why he was being moved.
Bulger arrived at the penitentiary on Monday after being processed at an Oklahoma City transfer site. He was convicted in 2013 for a slew of crimes, including being linked to at least 11 murders. He was serving a life sentence for 11 murders. Details of his death have not been announced. "No staff or other inmates were injured, and at no time was the public in danger".
Bulger, who served as the leader of Boston's violent Winter Hill Gang, was reportedly killed overnight at the maximum security prison, the Boston Globe reported, citing two individuals briefed on the incident. A character in the 2006 Martin Scorsese film "The Departed" was based on Bulger.
The blood-thirsty Bulger ran Boston's powerful Irish mob, the Winter Hill Gang, while simultaneously working as an informant for a local Federal Bureau of Investigation agent.More news: Saudi prosecutor in Khashoggi case meets Istanbul chief prosecutor: Anadolu
Bulger was born September 3, 1929, in Boston. He had a kind of Robin Hood-like image among some locals, but authorities said he would put a bullet in the brain of anyone who he even suspected of double-crossing him.
His legend only grew as Bulger evaded arrest after his disappearance, with endless reported sightings around the USA and the world.
In one of those heartfelt letters, Bulger wrote: 'My 16 years on the run were the happiest years of my life and hers.
"I thought they had him under a lot of security", she said.
She is now behind bars at a federal women's prison in Minnesota serving an eight-year sentence. According to the Boston Globe, a source said last week that Bulger was expected to be moved to a medical facility due to his deteriorating health.