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.
He had recently arrived to the prison after being moved from a prison in Florida to a transfer facility in Oklahoma City. A jury convicted Bulger of a host of federal charges in 2013, including being involved in the murders of Donahue and 10 others.
Bulger was organized crime boss of the Winter Hill Gang for a number of years and was second after terrorist Osama bin Laden the FBI Ten Most Wanted Fugitives list for 12 years. The Oklahoma facility he was held in last week often serves as a temporary stop for inmates in transit. A prison source told TMZ that Bulger was attacked by three fellow prisoners on Tuesday morning. 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.
In 2013, the notorious Boston mob boss was convicted of participating in 11 murders that spanned Florida, Oklahoma, and MA.
Bulger worked almost two decades as an Federal Bureau of Investigation informant while simultaneously running the Winter Hill Gang in Boston.
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.
"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.
She may not even know that her beau, the notoriously murderous "Whitey", is dead as she's now detained in Waseca, Minnesota serving out her eight-year jail sentence.
Prosecutors said that Greig had numerous opportunities to leave Bulger or report him, but she never did.
Bulger was accused of strangling Debra Davis, the 26-year-old girlfriend of his partner, Stephen "The Rifleman" Flemmi, and Deborah Hussey, also 26, the daughter of Flemmi's common-law wife.