He was discovered unresponsive in his cell at the maximum security facility shortly after being transferred there from a Florida jail.
Bulger was the head of a violent South Boston crime ring known as the Winter Hill Gang from the 1970s into the 1990s. In 1995, Bulger was tipped off about his imminent indictment by an FBI agent and escaped, remaining on the FBI’s 10 Most Wanted list until getting apprehended in 2011 in Santa Monica, California.
The former leader of South Boston's Winter Hill gang inspired the film Black Mass featuring Johnny Depp, and The Departed, which won the Academy Award for best picture in 2006.
His death comes on the same day that he was transferred to the Hazelton penitentiary in West Virginia, which houses 1,385 inmates, according to reports.
An inmate with mafia ties is now under investigation for Bulger's killing, three sources briefed on the incident told the Boston Globe.