NBC10 Boston has obtained the death certificate for Boston crime boss James "Whitey" Bulger.
The document, filed in Suffolk Superior Court, shows that Bulger's cause of death was "Blunt Force Injuries of the Head." The death certificate said the injury occurred when he was "Assaulted by other(s)" in his prison cell at the United States Penitentiary, Hazelton in Bruceton, West Virginia.
It also says he was found in his cell at 8:21 a.m. on Oct. 30, 2018.
His final manner of death is listed as homicide.
Six months after Bulger's murder, no one has been charged in connection with his death.
Federal law enforcement officials believe Bulger was killed with a "lock-in-the-sock" inside his cell, multiple law enforcement officials told NBC News. The "lock-in-the-sock" involves placing a prison lock inside an inmate's sock and hitting the victim repeatedly.
In this case, two sources said that Bulger was hit repeatedly.
Fotios "Freddy" Geas, a reputed New England mafia hitman, has been named as a potential suspect. But two sources told NBC News that Geas is one of several potential suspects they are looking at for the killing.
Bulger led the Irish mob in South Boston for decades and became an FBI informant who supplied information on the New England Mafia, his gang's main rival, in an era when bringing down the Italian mob was a top national priority for the bureau.
Tipped off that he was about to be indicted, Bulger became a fugitive and eluded authorities for 16 years before being captured in 2011. He was convicted in 2013 in 11 underworld slayings and a long list of other crimes and was sentenced to spend the rest of his life behind bars.
Bulger was killed just days after arriving at the high-security Hazelton prison. He had previously been in a prison in Florida, with a stopover at a transfer facility in Oklahoma City. Federal Bureau of Prisons officials and his attorney declined to comment on why he was being moved.
Bulger's attorney, J.W. Carney Jr., blamed his death on prison officials, saying Bulger "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."