CrimeMobster 'Whitey' Bulger's official cause of death revealed

© FBI/Zuma Press/TNS South Boston gangster James "Whitey" Bulger in an undated photo released in 1994 by the FBI. Bulger died in prison Tuesday.

James "Whitey" Bulger Jr., a cold-blooded mobster who was found dead in a West Virginia prison cell six months ago, died of "blunt force injuries of the head," according to a death certificate obtained Thursday by an NBC affiliate in Boston.

The document states the 89-year-old was assaulted inside his cell, where he was found shortly after 8 a.m. on Oct. 30, 2018. The manner of death was homicide, according to the document.

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Although the document provides the first official word on how Bulger died, law enforcement officials previously said his killers struck him with a padlock stuffed inside a sock and then wrapped him in a blanket to try to conceal the attack.

Bulger lived as a fugitive for 16 years and even topped FBI's most-wanted list before he was found living near in Santa Monica, Calif., in 2011. He was convicted two years later and was serving a life sentence for 11 murders, extortion and racketeering schemes that authorities said earned him more than $25 million.

The Irish American crime boss had been transferred to the high-security United States Penitentiary, Hazelton just a day before he was killed.

No one has been charged in his death, but a potential suspect is Fotios "Freddy" Geas, a Massachusetts hitman who reportedly hates "rats."

Bulger had a long and corrupt relationship with FBI agents before becoming a fugitive, often feeding them tips on other criminals.

His longtime girlfriend, Catherine Greig, is also behind bars for helping Bulger.

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