The cause of death of notorious Boston gangster James "Whitey" Bulger has been revealed. 

Bulger died of “blunt force injuries of the head," according to a death certificate obtained Thursday by an NBC affiliate in Boston. They reported 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.

Bulger was serving two life sentences after being convicted in 2013 of multiple crimes, including participating in 11 murders. One of them was Tulsa businessman Roger Wheeler, who was shot in the parking lot of Southern Hills Country Club in 1981.

Bulger was one of the FBI's most wanted fugitives for 16 years until his arrest in Santa Monica, California in 2011.