The US Justice Department indicated that Bulger had been found at roughly 8:20 a.m. local time on Tuesday and that prison staffers had attempted to resuscitate him.
Bulger was killed in the U.S. Penitentiary Hazelton in West Virginia, a maximum security prison, where he was transferred to shortly after spending time in an Oklahoma prison, according to the Boston Herald.
Bulger had just been moved to USP Hazelton, a high-security prison with an adjacent minimum security satellite camp in Bruceton Mills, West Virginia.
Sen. Joe Manchin, D-W.Va., had recently joined other officials in sending a letter to Attorney General Jeff Sessions voicing concerns over staffing levels at federal prisons, including Hazelton. At his ensuing trial, he was convicted of 11 murders in three states.
Bulger ran the Irish mob for decades and was on the run from authorities for 16 years - and on the FBI's 10 Most Wanted list for a dozen of those - before being arrested in Santa Monica in 2011. He was sentenced to serve two consecutive life sentences plus five years.
"Life-saving measures were initiated immediately by responding staff. Mr. Bulger was subsequently pronounced dead by the Preston County Medical Examiner", the U.S. Bureau of Prisons said in a statement. One tipster insisted spying Bulger inside a Boston movie theater in 2006, watching the Martin Scorsese movie "The Departed" - where Jack Nicholson played a mob boss based on the fugitive son of South Boston.
The feared former leader of the Winter Hill Gang, Bulger was convicted in 2013 of participating in 11 murders stretching from MA to Florida to Oklahoma.
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.
Bulger was born September 3, 1929, in Dorchester, Massachusetts.
"I heard he wasn't feeling well a while back", Jackie Bulger said of Whitey.
Greig is still serving her sentence at a federal prison in Minnesota.