(CNN) Notorious mob boss James "Whitey" Bulger was brutally beaten in the fatal attack Tuesday at the Hazelton prison in Bruceton Mills, according to law enforcement officials.

The South Boston mobster, 89, died a day after he was transferred there from another facility -- the victim of the kind of brutality he once handed out.

At the time of the killing, Bulger was in the general prison population, which gave inmates easy access to him, according to a federal law enforcement official, who has knowledge of the investigation.

Bulger was found unresponsive at 8:20 a.m., and was pronounced dead after failed lifesaving measures, prison officials said.

Investigators believe he was attacked by more than one person. At least one of the inmates involved has ties to organized crime in Massachusetts, the official said.

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