Federal law enforcement officials believe Boston crime boss James "Whitey" Bulger was killed with a "lock-in-the-sock" inside his cell, multiple law enforcement officials briefed on the case tell NBC News.
The "lock-in-the-sock" involves placing a prison lock inside an inmate's sock and hitting the victim repeatedly.
In this case, two sources say that Bulger was hit repeatedly.
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Investigators are still exploring several theories about who killed Bulger and when it happened.
Fotios "Freddy" Geas a reputed New England mafia hitman has been named as a potential suspect. But two sources tell NBC News that Geas is one of several potential actors they are looking at for the Bulger killing.
Bulger led the Irish mob in South Boston for decades and became an FBI informant who supplied information on the New England Mafia, his gang's main rival, in an era when bringing down the Italian mob was a top national priority for the bureau.
Tipped off that he was about to be indicted, Bulger became a fugitive and eluded authorities for 16 years before being captured in 2011. He was convicted in 2013 in 11 underworld slayings and a long list of other crimes and was sentenced to spend the rest of his life behind bars.
Bulger was killed Tuesday at the United States Penitentiary, Hazelton in Bruceton, West Virginia. He had just arrived Monday at the high-security prison. He had previously been in a prison in Florida, with a stopover at a transfer facility in Oklahoma City. Federal Bureau of Prisons officials and his attorney declined to comment on why he was being moved.
Bulger's attorney, J.W. Carney Jr., blamed his death on prison officials, saying Bulger "was sentenced to life in prison, but as a result of decisions by the Federal Bureau of Prisons, that sentence has been changed to the death penalty."
The investigation is ongoing.