(CNN) James "Whitey" Bulger, the notorious and much-feared former Boston mob boss, was killed by blunt force injuries to the head, according to his death certificate, which was made public Thursday.

Bulger, 89, was found unresponsive on October 30 at the US Penitentiary Hazelton in West Virginia, one day after being transferred there.

The death certificate lists the time as 8:21 a.m.

His injuries occurred when he was "assaulted by other(s)," the document says. The approximate time between onset of his injuries and death is listed as "minutes."

He was pronounced dead by the Preston County medical examiner after lifesaving measures failed, authorities have said. The pronouncement came at 9:04 a.m., the death certificate says.

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