No official details were given on the circumstances of Mr. Bulger’s death. But some of the federal prison employees, all of whom spoke on the condition of anonymity because the information was not yet public, said Mr. Bulger did not die of old age. Rather, they said, he was beaten to death by at least two fellow inmates at the West Virginia federal prison, where he had been transferred to the day before.

Inside a maximum security cell at the U.S. Penitentiary, Hazelton in Bruceton Mills, W. Va., Mr. Bulger was likely attacked early in the morning — sometime between 6 a.m., when cell doors were unlocked so inmates could go to breakfast, and 8 a.m., when the staff made rounds, federal prison employees said. Mr. Bulger had not emerged from his cell for breakfast and when prison staff went to check on him, they found him wrapped in blankets and unresponsive, they said.

When they tried to shake him, blood spattered the floor. He was dead.

One of the federal prison workers said that the inmates involved in the killing were thought to be “affiliated with the mob.” Separately, a senior law enforcement official, who oversees organized crime cases but was not involved in the investigation into Mr. Bulger’s death, said he was told by a federal official that an organized crime figure was believed to be responsible.

And a lawyer for Mr. Bulger, J.W. Carney Jr., made clear that he blamed the prison system for the death of his client, whom, he said, he was proud to have been appointed to represent. “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,” Mr. Carney said.

Mr. Bulger is by no means the first organized crime figure to be killed once arrested, tried, convicted and sent off to serve his time.