The Wall Street Journal reports the family is seeking $200 million in damages in the forthcoming claim. They say Bulger was "deliberately placed in harm's way" when he was transferred to a West Virginia prison and placed in the general inmate population hours before his death.
The 89-year-old died of blunt force injuries to the head. No charges have been filed, but officials have said two Massachusetts mobsters are suspects.
The federal Bureau of Prisons didn't respond to an email seeking comment on Friday. Bulger's lawyer also didn't respond.
Bulger was sentenced to life in prison for his role in 11 gangland killings. He'd also been an FBI informant.
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