The official told The Associated Press on Wednesday that Fotios “Freddy” Geas and at least one other inmate are believed to have been involved in 89-year-old Bulger’s killing Tuesday at a West Virginia prison.
The longtime investigator spoke on the condition of anonymity because he was not authorized to talk about the case.
Geas was convicted in the 2003 killing of western Massachusetts mobster Adolfo Bruno.
Bulger was a longtime informant for the FBI who provided information on the Mafia. Geas was known to despise gangsters who ratted each other out.
Attorney David Hoose, who initially represented Geas in the Bruno case, says Geas “did not and would not rat on anyone.”
Federal officials say they are investigating Bulger’s death as a homicide.