April 25 (UPI) -- Boston mob boss James "Whitey" Bulger was killed by blunt force injuries to his head, his death certificate indicates.
Bulger's death certificate, which was released to the public Thursday, said he was assaulted by others in his prison cell at the United States Penitentiary, Hazelton in Bruceton, W.Va., on Oct. 30.
The final manner of death was listed as homicide and federal law enforcement officials told NBC News they believe an inmate beat Bulger repeatedly with a prison lock placed inside of a sock.
Bulger, 89, was pronounced dead by the Preston County medical examiner at 9:04 a.m., but the documents listed his time of death as 8:21 a.m., minutes after the onset of his injuries.
No one has been charged in the case of Bulger's death, but mobsters Paul J. DeCologero and Fotios "Freddy" Geas have been named as suspects.
Bulger's death came less than 11 hours after he was transferred to the West Virginia prison where Geas and DeCologero were also being held.
The two men were notorious Massachusetts mobsters, who may have been motivated to kill Bulger for his work as a secret FBI informant while he led South Boston's Winter Hill Gang.
At the time of his death, Bulger was five years into two life sentences for the murders of 11 men and women in Massachusetts, Oklahoma and Florida.