CLARKSBURG, W.Va. — A former Mafia hitman is set to be sentenced in the fatal prison bludgeoning of notorious Boston gangster James “Whitey” Bulger after making a deal with prosecutors to change his plea from not guilty.

Federal prisoner Fotios “Freddy” Geas is scheduled to appear Friday in U.S. District Court in northern West Virginia.

Prosecutors said Geas used a lock attached to a belt to repeatedly hit the 89-year-old Bulger in the head hours after Bulger arrived at the U.S. Penitentiary, Hazelton in West Virginia from another lockup in Florida in October 2018.

Bulger, who ran the largely Irish mob in Boston in the 1970s and ’80s, served as an FBI informant who ratted on his gang’s main rival, according to the bureau. Bulger strongly denied ever being a government informant.

Bulger became one of the nation’s most wanted fugitives after fleeing Boston in 1994. He was captured at age 81 after more than 16 years on the run and convicted in 2013 in a string of 11 killings and dozens of other gangland crimes.

Geas, whom authorities say was a Mafia hitman, is already serving a life sentence for previous violent crimes. He was charged with murder and conspiracy to commit first-degree murder in Bulger’s death, which each carry a sentence up to life. Last year the Justice Department said it would not seek the death penalty.

It’s unclear from court filings how Geas will plead, but the court scheduled the sentencing for the same plea hearing. Plea deals for Geas and two other Hazelton inmates were disclosed May 13, and an attorney for Geas did not oppose the government’s motion.

Another prisoner, Massachusetts gangster Paul J. DeCologero, was sentenced to more than four years in prison in August on an assault charge. Prosecutors said he acted as a lookout while Geas beat Bulger. A third inmate, Sean McKinnon, pleaded guilty in June to lying to FBI special agents and was given no additional prison time.

An inmate witness told a grand jury that DeCologero told him Bulger was a “snitch” and they planned to kill him as soon as he came into their unit.