10:25 AM PT: An official from the Bureau of Prisons says at approximately 8:20 AM Bulger was found unresponsive, adding "Life-saving measures were initiated immediately by responding staff." He was later pronounced dead by the local medical examiner.
The Federal Bureau of Investigation was notified and an investigation has been initiated. The BOP official said no staff or other inmates were injured, and at no time was the public in danger.
James "Whitey" Bulger, the former South Boston mob boss and longtime FBI informant, was killed while serving a life sentence inside a federal prison.
89-year-old Bulger, who was convicted for the murder of 11 people, was just transferred to the high-security USP Hazelton in West Virginia.
The president of the union representing the workers at Hazelton said he was told that a male inmate was slain overnight, but was not informed of the identity of the prisoner.
NBC News reported Tuesday morning that the slain inmate was in fact Bulger, although there are no details on how the alleged murder went down.
In 2013, a federal jury convicted him of participating in 11 murders throughout the '70s and '80s. He was captured two years earlier hiding out in an apartment in Santa Monica, CA after being on the run for more than 16 years.