Fotios Geas, 55; Paul J. DeCologero, 48; and Sean McKinnon, 36, were all charged with conspiracy to commit first-degree murder on Wednesday (August 17), the U.S. Attorney's Office for Northern West Virginia confirmed in a statement obtained by NBC News.
Geas and DeCologero were also charged with aiding and abetting first-degree murder and assault resulting in serious bodily injury, while McKinnon was charged with making false statements to a federal agent.
Bulger was found dead in his U.S. Penitentiary Hazelton cell by a prison guard at around 8:21 a.m. on October 30, 2018, less than 12 hours after he was transferred to the Bruceton Mills, West Virginia facility from Florida.
Geas and DeCologero are accused of striking Bulger multiple times in the head, which lead to his death, according to authorities.
Geas and DeCologero were both incarcerated when charged on Wednesday.
McKinnon, who was released from Hazelton in February, was taken into custody by the FBI in Ocala, Florida, an FBI spokesperson confirmed via NBC News.
Gaes, who is serving a life sentence for murder, was described by authorities as the central suspect in relation to the Bulger's death.
Federal prosecutors said that Gaes was an enforcer for the New England mafia in the 1990s and 2000s and a rival of Bulger, who led the Winter Hill Gang -- a confederation often referred to as an Irish Mob organization -- and was a secret FBI informant.
McKinnon was Geas' cellmate and serving an eight-year term for selling guns from a Vermont firearms store at the time of the incident.
All three suspects were placed in solitary confinement in the hours after Bulger was found dead inside his cell.