The family of notorious Boston mobster James “Whitey” Bulger plans to file a lawsuit against the government claiming wrongful death and negligence more than a month after his mysterious murder inside a West Virginia prison.

Hank Brennan, Bulger’s longtime attorney, told the Wall Street Journal on Sunday, the gangster’s estate wants to find out why authorities sent the infamous mob boss to U.S. Penitentiary Hazelton in October, where he was beaten to death inside his unlocked cell fewer than 24 hours after his transfer.

“It’s important for the family and the public to know why the prisons decided to wheel an 89-year-old man with a history of heart attacks into one of the most dangerous prisons in the country,” Brennan told the Journal.

WHITEY BULGER, INFAMOUS BOSTON MOB BOSS, KILLED IN PRISON, MULTIPLE REPORTS SAY

Brennan, who became Bulger’s attorney after the gangster’s arrest in 2011, said Bulger told him in September during one of their last conversations that he was getting out of solitary confinement at a federal prison in Florida and being transferred to a prison medical facility.

Instead, a month later, on Oct. 30, the 89-year-old mob boss was killed the day he arrived at the West Virginia prison. He was found bloodied and wrapped in a blanket after apparently being beaten with a lock stuffed in a sock.

No one has been charged yet in the killing, but two Massachusetts mobsters are among the suspects, officials have said.

A Bureau of Prisons spokesperson told the WSJ that Bulger was transferred from the Florida prison because of a threat he made against a staff member and the transfer was made in accordance with the bureau’s policy.

Brennan disputes the threat allegation.

WHITEY BULGER MURDER THEORY EMERGES DAYS AFTER PRISON DEATH

Bulger, the model for Jack Nicholson’s ruthless crime boss in the 2006 Martin Scorsese movie “The Departed,” led a largely Irish mob that ran loan-sharking, gambling and drug rackets. He was also an FBI informant who ratted on the New England mob, his gang’s main rival, in an era when bringing down the Mafia was a top national priority for the FBI.

He spent 16 years as one of America's most wanted fugitives until he was found in 2011, living with his girlfriend in a rent-controlled apartment in Santa Monica, California.

Bulger was sentenced to life in prison in 2013 for 11 murders and numerous other crimes. He'd been assigned to prisons in Arizona and Florida specializing in sick inmates before he arrived at Hazelton prison in Bruceton Mills last month.

The Associated Press contributed to this report.