Whitey Bulger died brutally .

The notorious South Boston mob boss -- who was portrayed by Johnny Depp in Black Mass and (loosely) by Jack Nicholson in The Departed -- was beaten to death not long after he was transferred to Hazelton prison in West Virginia.

After being on the run for years, Bulger was caught in 2011 and convicted of 31 counts, including multiple murders.

Bulger's killers, two inmates, walked into his cell at 6 AM. They found 89-year-old in a wheelchair and beat him with a lock in a sock. Then they used chivs to try to remove his eyes and his tongue. The killers returned to the cell hours later and cleaned it up.

Bulger had been an FBI informant for two decades while serving as the head of the Winter Hill Gang. His deal with the Feds protected him from prosecution as long as he gave the FBI information about rival gangsters. 

The removal of Bulger's eyes and tongue was in retaliation for him being a rat.

Officials believe 51-year old Fotios “Freddy” Geas was behind and involved with the prison hit on Bulger. 

Geas is serving a life sentence at Hazelton for the 2003 killing of mob boss Adolfo “Big Al” Bruno and his associate Gary Westerman. He's known for his disdain of rats. 

“He’s a rich man now," a law enforcement source said of Geas. "He’ll run any prison he’s in."