BOSTON — A Mafia hit man who is said to hate "rats" is under suspicion in the slaying of former Boston crime boss and longtime FBI informant James "Whitey" Bulger. The former mob boss was found dead just hours after he was transferred to a West Virginia prison, a former investigator briefed on the matter said Wednesday.

The official said that Fotios "Freddy" Geas and at least one other inmate are believed to have been involved in Bulger's killing. The longtime investigator was not authorized to discuss the case and spoke on condition of anonymity. People familiar with the investigation also told The Boston Globe that Geas was a suspect in Bulger's killing.

Authorities have not disclosed the cause of death for the 89-year-old Bulger. He was found dead Tuesday.

In this April 14, 2009 photo, Fotios "Freddy" Geas appears for a court proceeding in his defense in the Al Bruno murder case, in Springfield, Mass. Geas and at least one other inmate are being investigated as suspects in the slaying of former Boston crime boss James "Whitey" Bulger, who was killed behind bars on Tuesday, Oct. 30, 2018, less than 24 hours after being transferred to a federal prison in West Virginia, according to a former investigator briefed on the matter. Don Treeger /The Republican via AP

Geas, 51, 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 who was gunned down in a Springfield, Massachusetts, parking lot.

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Private investigator Ted McDonough, who knew Geas, told The Boston Globe: "Freddy hated rats."

"Freddy hated guys who abused women. Whitey was a rat who killed women. It's probably that simple," McDonough told the newspaper, which first reported that Geas was under suspicion.

Geas, his brother, Ty Geas, and New York mob boss Artie Nigro were convicted in a case hung largely on the testimony of several gangsters turned government informants, reports MassLive. The brothers reportedly refused to cooperate as co-conspirators who did received lighter sentences. A one-time lawyer for Geas, Daniel D. Kelly, told MassLive Geas staunchly refused to testify against others: "He didn't even blink an eye. He didn't flinch. He just said no."

It was not clear whether Geas has a current attorney. Several other lawyers who represented him over the years didn't immediately respond to a request for comment. An FBI spokeswoman in Pittsburgh declined to comment on Geas. Federal officials said only that they are investigating the death as a homicide.

James "Whitey" Bulger, pictured after his arrest in 2011 CBS Boston

Law enforcement sources tell CBS News Bulger was apparently severely beaten by one or more inmates. According to the Boston Globe, Bulger's eyes were nearly gouged out.

For many victims of Bulger's reign of terror, his violent death seemed all too fitting.

"He deserved a slow death and that's what I hope he got," said Tommy Donahue whose father was killed by Bulger. Added his widow, Patricia, "I'm gonna buy myself a bottle of champagne, and I'm going to pop that cork."

Bulger led South Boston's Irish mob for decades and became an FBI informant who supplied information on the New England Mafia, his gang's main rival, in an era when bringing down the Italian mob was a top national priority for the bureau.

Tipped off that he was about to be indicted, Bulger became a fugitive and eluded authorities for nearly two decades before being captured in 2011. He was convicted in 2013 in 11 underworld slayings and a host of other crimes and was sentenced to spend the rest of his life behind bars.

He had just arrived Monday at USP Hazelton, a high-security prison in Bruceton Mills, West Virginia. He had previously been in a prison in Florida, with a stopover at a transfer facility in Oklahoma City. Federal Bureau of Prisons officials and his attorney declined to comment on why he was being moved.

Bulger's attorney, J.W. Carney Jr., blamed his death on prison officials, saying Bulger "was sentenced to life in prison, but as a result of decisions by the Federal Bureau of Prisons, that sentence has been changed to the death penalty."

Bureau of Prison officials had no comment on Carney's remarks.

The Geas brothers were not made members of the Mafia because they were Greek, not Italian. But they were close associates of the mob and acted as enforcers.