James 'Whitey' Bulger, who lived a double life as one of Boston's most notorious gangsters and a secret FBI informant, has been killed in prison.
The 89-year-old was found dead in his cell after being transferred to a federal prison, according to US reports.
He had recently been transferred from Florida to the high-security Hazelton prison in West Virginia.
The cause of his death has not yet been confirmed.
Bulger, the former leader of South Boston's Winter Hill Gang, was serving life in prison.
He was portrayed by Johnny Depp in the film Black Mass, and inspired Martin Scorsese's The Departed, which won the Academy Award for best picture in 2006.
Bulger led a largely Irish mob that ran loan-sharking, gambling and drug rackets in the Boston area. He also served as an FBI informant who ratted on his gang's main rival.
The Winter Hill gang operated for more than two decades, while an FBI agent looked the other way when it came to Bulger's crimes in return for information on other gangsters.
Bulger was feared for his short temper and brutality. Prosecutors said he strangled two women with his hands and tortured a man for hours before shooting him in the head with a machine gun.
"We took what we wanted," Kevin Weeks, a former Bulger lieutenant who would eventually testify against him, wrote in his memoir, Brutal.
"We made millions through extortion and loansharking and protection. And if someone ratted us out, we killed him. We were not nice guys."
Bulger became one of the nation's most-wanted fugitives after fleeing Boston in late 1994. After more than 16 years on the run, he was captured aged 81 in Santa Monica, California, where he had been living with his longtime girlfriend, Catherine Greig.
He had gone to great lengths to avoid prison, including killing potential witnesses, but it all ended with a tip from a former Icelandic beauty queen.
He was finally arrested in June 2011. In 2013, he was convicted of participating in 11 murders in the 1970s and 1980s and was sentenced to two consecutive life sentences.
