Sources at the USP Hazelton prison in West Virginia tell TMZ, Bulger was killed in his cell at around 6 AM Tuesday, NOT in a common area as first reported. We're told 4 inmates walked into the cell where Whitey was sitting in his wheelchair.

They brutally beat him with a lock in a sock and used a shiv to try and gouge his eyes out. They also attempted to cut out his tongue. We're told there was a lot of blood in Bulger's mouth but we don't know if the tongue was actually removed.

We're told surveillance cameras captured the 4 inmates walking into Bulger's cell and walking out with their clothes clearly bloodied. The attack was not caught on video.

We're told the killers went back to their cells, changed out of their bloody clothes and then walked back to Bulger's cell with a mop and a bucket to clean up the mess.

As for the motive ... our prison sources say prison officials believe it was mob retaliation because Bulger was considered a rat and an informant who helped put away several members of the mob.

The officials believe the mob put out a hit on Bulger and removal of the eyes and tongue are signs of such a hit.

BTW ... Patricia Donahue -- the widow of Michael Donahue whom Bulger killed in 1982 -- reacted to the news by saying justice was served. She also told us how she plans to celebrate. Her son, Tommy, agreed, saying how Bulger's death brought some much-needed relief.