(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.
Bulger, who eluded federal authorities for more than 16 years before his arrest in June 2011, was serving the rest of his life in prison for a litany of crimes that included his role in 11 murders.
He was sentenced in November 2013 to two life terms plus five years after being an architect of a criminal enterprise that, in the words of a federal judge, committed "unfathomable" acts that terrorized a city.
A federal jury convicted Bulger that year of 31 counts, including racketeering, extortion, money laundering, drug dealing and weapons possession.
The jury found him culpable in 11 killings from 1973 through 1985.