The report said he was found unresponsive at 8:20 a.m. and was pronounced dead by the Preston County medical examiner.
Whitey Bulger - one of America's most notorious mobsters - was found dead Tuesday in a federal prison where he had just been transferred.
"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", defense attorney J.W. Carney Jr. told CNN.
He had been in a prison in Florida before a stopover at a transfer facility in Oklahoma City.
Prosecutors believe Bulger was responsible for more crimes during the 1970s, 80s and 90s, while he oversaw his criminal empire, than he was convicted of.
As Radar previously reported, the former Boston-based crime boss was finally captured by authorities in 2011, after more than 16 years on the run.
Bulger was sentenced to life in prison for 11 murders.
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. His dark life story also inspired several other films, such as The Departed, which won the Academy Award for Best Picture in 2006.
But by 1995, even Bulger's friends in high places couldn't keep him from an indictment - this time for obstruction of justice, racketeering, drug dealing and extortion.
Bulger was born September 3, 1929, in Boston. But in his case, it was never clear who was informing to who.
She is now behind bars at a federal women's prison in Minnesota serving an eight-year sentence.
"I thought they had him under a lot of security", she said.
The gangster was arrested with girlfriend Catherine Greig, now 67.