BOSTON >> Three men, including a Mafia hitman, have been charged in the 2018 killing of notorious Boston crime boss James “Whitey” Bulger, the Justice Department said Thursday.

The charges against Fotios “Freddy” Geas, Paul J. DeCologero and Sean McKinnon come nearly four years after Bulger’s killing, which raised questions about why the known “snitch” was placed in the West Virginia prison’s general population instead of more protective housing. The men were charged with conspiracy to commit first-degree murder.

Authorities have not revealed a possible motive for Bulger’s killing in October 2018, which came hours after he was transferred to USP Hazelton in West Virginia from a prison in Florida. He had been serving a life sentence for 11 murders and other crimes. Prosecutors allege Geas and DeCologero struck Bulger in the head multiple times, causing his death.

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. He later became one of the nation’s most-wanted fugitives. Bulger strongly denied ever being a government informant.

The Justice Department has also charged Geas, 55, and DeCologero, 48, with aiding and abetting first-degree murder, along with assault resulting in serious bodily injury. Geas faces a separate charge of murder by a federal inmate serving a life sentence, and McKinnon, 36, is charged separately with making false statements to a federal agent.

Geas and DeCologero were identified as suspects shortly after Bulger’s death, according to law enforcement officials at the time, but they remained uncharged as the investigation dragged on for years.

All three were placed in solitary confinement throughout the probe, family members told The Boston Globe. McKinnon’s mother told the newspaper that her son, who was Geas’ cellmate at the time of Bulger’s killing, told her he didn’t know anything about the slaying.

Daniel Kelly, an attorney for Geas, said Thursday that the charges aren’t a surprise, but don’t justify his client’s continued placement in solitary confinement.

Emails seeking comment were sent Thursday to an attorney for Bulger’s family. It wasn’t immediately clear if McKinnon and DeCologero had attorneys to comment on their behalf.

Geas remains in prison in Hazelton. DeCologero is being held in another federal prison facility. McKinnon was released from prison last month after pleading guilty in 2015 to stealing guns from a firearms dealer. He was on federal supervised release when the indictment was handed down, and was arrested Thursday in Florida.

Bulger was the third inmate killed in six months at USP Hazelton, where workers and advocates had long warned of dangerous conditions at the penitentiary nicknamed “Misery Mountain.”

Bulger’s family sued the Federal Bureau of Prisons and 30 unnamed employees of the prison system over his death, alleging it appeared the gangster was “deliberately sent to his death.” A federal judge dismissed the family’s lawsuit in January.

Bulger fled Boston in late 1994 after his FBI handler, John Connolly Jr., warned him he was about to be indicted.

After more than 16 years on the run and with a $2 million reward on his head, he was captured at age 81 in Santa Monica, California, where he had been living in a rent-controlled apartment near the beach with his longtime girlfriend, Catherine Greig.

He was convicted in 2013 of the 11 slayings, as well as extortion and money-laundering after a sensational racketeering trial.

His transfer to Hazelton was prompted by disciplinary issues, a federal law enforcement official told The Associated Press in 2018. The official insisted on anonymity because they were not authorized to release details. In February 2018, Bulger threatened an assistant supervisor at the prison in Florida, telling her “your day of reckoning is coming.”

A prison workers’ union official told the AP that year that sending Bulger to the troubled federal penitentiary that housed other New England gangsters was like giving him a “death sentence.”

But Bulger never admitted to working for the FBI. Court papers made public in the civil case brought by his family showed that he was interviewed by staff after arriving at Hazelton about whether there were reasons he should be kept out of the general population. An intake screening form signed by Bulger said he answered “no” to questions such as “have you assisted law enforcement agents in any way?”

DeCologero was part of an organized crime gang led by his uncle on Massachusetts’ North Shore called the “DeCologero Crew.”

He 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 was a close associate of the Mafia and acted as an enforcer, but was not an official “made” member because he’s Greek, not Italian.

Geas 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. Another mobster ordered Bruno’s killing because he was upset he had talked to the FBI, prosecutors said.