The 89-year-old Bulger had recently been moved from a prison in Florida to a transfer facility in Oklahoma City.
The Bureau of Prisons declined to comment last week on why Bulger - who was serving a life sentence - was being moved.
After a sensational, months long trial in federal court in Boston in 2013, a jury convicted Bulger of 11 murders in the 1970s and 1980s while running a national criminal enterprise that collected millions of dollars from gambling, extortion, and drug trafficking.
The gangster inspired Jack Nicholson's character Francis "Frank" Costello in the Oscar-winning film The Departed. "No staff or other inmates were injured, and at no time was the public in danger".
Bulger worked almost two decades as an Federal Bureau of Investigation informant while simultaneously running the Winter Hill Gang in Boston. He was convicted in 2013 for playing a role in 11 murders.More news: Migrant convoy swells to 5000, heads to US
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Bulger was later identified as the dead man, although officials released no details surrounding his sudden, violent demise.
Bulger would pen notes on his lawyer's legal pad at his cell in MA back in 2014 and have them delivered via his lawyer to Greig who was detained at a Rhode Island Jail at the time. The legendary crime boss remained on the FBI's Most Wanted List for 12 of the years he was on the lam. A high school dropout, Bulger began a life of crime that eventually led to prison stays at Alcatraz and Leavenworth. But in his case, it was never clear who was informing to who.
"I heard he wasn't feeling well a while back", Jackie Bulger said of Whitey.
Then they were busted in Santa Monica in 2011, they were found with $800,000 in cash and had 30 weapons stashed in the rental home.
She remains locked up at a women's prison in Minnesota.