James “Whitey” Bulger, the South Boston mobster who was captured after years on the run, was killed in a West Virginia federal prison by at least two inmates, according to two Federal Bureau of Prisons employees who spoke on the condition of anonymity because the information was not yet public.

Bulger, 89, had been transferred to the U.S. Penitentiary, Hazelton in Bruceton Mills, West Virginia, on Monday and was beaten to death shortly after his arrival, according to two of the prison workers. One of the workers said the inmates were thought to be “affiliated with the mob,” but did not know the specifics of the association.

A senior law enforcement official who oversees organized crime cases but was not involved in the investigation into Bulger’s death, said he was told by a federal law enforcement official that an organized crime figure was believed to be responsible for the killing.

A prison worker said Bulger, who had been serving a life sentence for 11 murders, had been transferred to the Hazelton prison after he had threatened a staff member at the Coleman prison complex in Sumterville, Florida.

The death of Bulger was announced by the Federal Bureau of Prisons on Tuesday, but a cause was not provided. “The Federal Bureau of Investigation was notified and an investigation has been initiated,” the news release said. “No staff or other inmates were injured, and at no time was the public in danger.”

Bulger had just been moved to the West Virginia penitentiary. It was the latest in a series of prison transfers for him. He had been incarcerated in Arizona, Oklahoma and Florida, prison officials have said, without giving reasons for the moves.

A shortage of correctional officers has become chronic under President Donald Trump, leaving some prison workers feeling ill-equipped and unsafe on the job, according to a New York Times investigation published this year. Some prisons are so pressed for correctional officers that they regularly compel teachers, nurses, secretaries and other support staff to step in.

The Hazelton prison in particular has been plagued by violence. The prison has regularly assigned support staff to guard duty since mid-2016, though it recently tried to curtail the practice. Last year, The Times found, the prison had 275 violent episodes, including fights among inmates and major assaults on staff, an almost 15 percent increase from 2016. In April, an inmate was killed in a fight. Local news media reported that an altercation between inmates led to another inmate’s death last month.

Sen. Joe Manchin, D-W.Va., had recently joined other officials in sending a letter to Attorney General Jeff Sessions voicing concerns over staffing levels at federal prisons, including Hazelton. “Unfortunately, our states, West Virginia and Pennsylvania, have seen firsthand how dangerous continual understaffing can be to both BOP staff and the inmates they supervise,” he wrote.