Multiple media reports indicate he was killed.
The Boston Globe reported that he was slain and that a fellow inmate with Mafia ties was being investigated for the slaying.
"The testimony of human suffering that you and your associates inflicted on others was at times agonizing to hear and painful to watch", US District Judge Denise Casper said at the hearing. There has been no information released about a suspect in the killing.
Officials found Bulger unresponsive around 8:20 a.m. He was pronounced dead by the county medical examiner. He was sentenced to serve two consecutive life sentences plus five years.More news: Interim CBI chief's orders including mass transfers on hold
He had been held in a high-security prison in the federal correctional complex at Coleman, Fla. He had been incarcerated in Arizona, Oklahoma and Florida, prison officials have said, without giving reasons for the moves.
Bulger had just been moved to Hazelton Penitentiary in West Virginia, known as one of the roughest prisons in the country. Even the city's most powerful crime family, the Angiulos, would go to prison, based on information Bulger gave to the Federal Bureau of Investigation.
Bulger would eventually become an informant for the Federal Bureau of Investigation, something he always denied, giving up information on his so-called friends that put them in jail while Whitey rose in the ranks of Boston's organized crime. He had a kind of Robin Hood-like image among some locals, but authorities said he would put a bullet in the brain of anyone who he even suspected of double-crossing him.
The trial came almost 20 years after he went on the run after being tipped off to a pending federal indictment by a corrupt FBI agent.
He was finally busted with longtime girlfriend Catherine Grieg in their West Coast apartment.