The FBI is investigating the death of notorious Boston gangster James “Whitey” Bulger.

Sources familiar with the incident told the Boston Globe an inmate with Mafia ties is being investigated in what could be a homicide.

Bulger was found unresponsive at the U.S. Penitentiary, Hazelton in West Virginia, and was pronounced dead a short time later. The 70s and 80s mob boss was serving life in prison for a long list of crimes including racketeering and extortion, as well as his role in 11 murders.

Bulger was 89 years old.