The man accused of acting as lookout during the prison killing of notorious Boston gangster James “Whitey” Bulger was sentenced to time served Monday after pleading guilty to a charge of lying to federal agents.

Sean McKinnon was accused along with two other inmates in the 2018 killing at a troubled West Virginia prison.

The other two inmates, Fotios “Freddy” Geas and Paul J. DeCologero, are accused of repeatedly hitting Bulger in the head within hours of Bulger being transferred to the prison.

Bulger, who ran the largely Irish mob in Boston in the 1970s and ’80s, 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.

DeCologero, who was in an organized crime gang led by his uncle in Massachusetts, was convicted of buying heroin that was used to try to kill a teenage girl his uncle wanted dead because he feared she would betray the crew to police. The heroin didn’t kill her, so another man broke her neck, dismembered her and buried her remains in the woods, court records say.

Geas, a Mafia hitman, and his brother were sentenced to life in prison in 2011 for their roles in several violent crimes, including the 2003 killing of Adolfo “Big Al” Bruno, a Genovese crime family boss in Springfield, Massachusetts.

Author Casey Sherman interviewed Gaes for his book “Hunting Whitey.”

“Freddy Geas was an old-school gangster, and he lived by the code that you don’t — quote, unquote — rat on your friends,” Sherman told CBS Boston.

He said Bulger should never had been transferred to the prison where he died because he was a known FBI informant.

“It’s the most violent prison in the federal prison system,” Sherman said.