What to Know Notorious Boston gangster James "Whitey" Bulger was found dead at a West Virginia prison on Tuesday, federal sources told NBC News.

Bulger, the head of Boston's Irish mob and an FBI informant, was serving a life sentence after being convicted in 2013 of 11 murders.

His case was the basis for the 2015 Johnny Depp film "Black Mass," based on the book by Boston Globe reporters Dick Lehr and Gerard O'Neill.

Notorious Boston gangster James "Whitey" Bulger was killed inside a federal prison in West Virginia on Tuesday.

NBC News is reporting that investigators are probing whether he was beaten to death by another inmate or inmates, according to multiple senior law enforcement officials.

Bulger, 89, had just been moved on Monday to the United States Penitentiary in Hazelton, a high-security prison with 1,270 male inmates and an adjacent minimum security satellite camp in Bruceton Mills, West Virginia.

"On Tuesday, October 30, 2018, at approximately 8:20 a.m., inmate James Bulger was found unresponsive at the United States Penitentiary (USP) Hazelton, in Bruceton Mills, West Virginia," the Federal Bureau of Prisons said in a statement. "Life-saving measures were initiated immediately by responding staff. Mr. Bulger was subsequently pronounced dead by the Preston County Medical Examiner."

The FBI and the US Attorney's office in the Northern District of West Virginia has been notified and an investigation has been initiated, the statement said. No staff or other inmates were injured.

Bulger, the head of Boston's Irish mob and an FBI informant, was serving a life sentence after being convicted in 2013 of a litany of gangland crimes in the 1970s and '80s, including participating in 11 murders. He was one of the FBI's most wanted fugitives for 16 years until his 2011 arrest in Santa Monica, California.

James “Whitey” Bulger, one of the most infamous killers in the country, eluded police for years before his capture. He was killed Tuesday in prison.

Bulger had recently been moved from a prison in Florida to a transfer facility in Oklahoma City. A spokesperson there said he was at the Oklahoma transfer center until Monday, according to a spokesman. Prior to that, he was held at a prison in Tucson, Arizona.

Bureau of Prisons officials and his attorney declined last week to comment on why he was moved.

Bulger's attorney. J.W. Carney Jr. issued this statement on Bulger's death:

"I was proud to be appointed by the Federal Court to represent James Bulger. He 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."

Bulger is now the third inmate killed at the federal prison in Bruceton Mills this year.

Earlier this month, a Congresswoman called for a federal investigation into what she described as “appalling conditions” at Hazelton.

In a letter to the Department of Justice (DOJ) Inspector General, Rep. Eleanor Holmes Norton (D-D.C.) wrote, “I believe that the federal employees serving in this facility have likely received inadequate training, are under-supported, and are being compelled to perform duties outside the scope of their positions and their training, which is leading to these horrific and entirely unacceptable outcomes.”

That letter came after two inmates died during violent struggles inside the prison.

Former Boston mob boss James ‘Whitey’ Bulger is the third inmate to be killed at USP Hazelton in West Virginia this year.

Last week, the president of the union representing corrections officers at the facility expressed concern about increasing workplace assaults.

Richard Heldreth, president of Local 420 of the Federation of Government Employees, told a newspaper that Hazelton is “dangerously understaffed.”

The most recent DOJ prison death statistics show there are about 10 homicides per year nationwide, according to figures reviewed by NBC10 Boston. With three inmates already killed this year, that would put Hazelton far above the norm.

There was plenty of reaction to Bulger's death in the Boston area.

Among the 11 victims Bulger was convicted of killing was Michael Donahue. Patricia Donahue, Michael's widow, said her husband was killed 36 years ago giving a friend a ride home. She said she doesn’t believe in the death penalty so she was content to have Bulger rot in prison.

"I believe that you die the way you live," Patricia Donahue said. "He killed a lot of people and he was killed, so that says a lot to me."

Steve Davis, the brother of Debra Davis who was another one of Bulger's victims, told NBC10 Boston his phone has not stopped ringing since the news of Bulger's death. He said Bulger's death is the closest he and his family have ever been to justice, although the relief doesn't erase the pain for any of the victims.

"There's never going to be closure to the heart of the victims and the families. It's never going to be closure for them," said Davis. "We lost something we can't get back. We lost a life."

The families of victims allegedly killed by James ‘Whitey’ Bulger are reacting to his death.

Bulger's story was the basis for the 2015 Johnny Depp film "Black Mass." and he was also the model for Jack Nicholson's ruthless crime boss in the 2006 Martin Scorsese movie, "The Departed." His character led a largely Irish mob that ran loan-sharking, gambling and drug rackets. Bulger also was 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.

Bulger fled Boston in late 1994 after his FBI handler, John Connolly Jr., warned him he was about to be indicted. With a $2 million reward on his head, Bulger became one of the FBI's "Ten Most Wanted" criminals, with a place just below Osama bin Laden.

Tom Duffy, a retired state police detective who searched for Bulger and was a consultant on "The Departed," called word of Bulger's death "celebratory news."

When the extent of his crimes and the FBI's role in overlooking them became public in the late 1990s, Bulger became a source of embarrassment for the FBI. During the years he was a fugitive, the FBI battled a public perception that it had not tried very hard to find him.

[NATL] The Life and Times of Mobster Whitey Bulger

After more than 16 years on the run, Bulger 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.

In 2013, he was convicted in the slayings, as well as extortion and money-laundering, after a sensational racketeering trial that included graphic testimony from three former Bulger cohorts: a hit man, a protege and a partner. He was sentenced nearly five years ago to two consecutive life sentences plus five years.

Bulger, nicknamed "Whitey" for his bright platinum hair, grew up in a gritty South Boston housing project and became known as one of the most ruthless gangsters in Boston. His younger brother, William Bulger, became one of the most powerful politicians in Massachusetts, leading the state Senate for 17 years.

While South Boston may have changed dramatically in the 25 years since James “Whitey” Bulger fled, his notorious legacy still lingers over many in the neighborhood.

In working-class "Southie," Bulger was known for helping old ladies across the street and giving turkey dinners to his neighbors at Thanksgiving. He had a kind of Robin Hood-like image among some locals, but authorities said he would put a bullet in the brain of anyone who he even suspected of double-crossing him.

"You could go back in the annals of criminal history and you'd be hard-pressed to find anyone as diabolical as Bulger," said Duffy.

"Killing people was his first option," Duffy said after Bulger was finally captured in 2011. "They don't get any colder than him," Duffy said after Bulger was finally captured in June 2011."

Bulger was accused of strangling Debra Davis, the 26-year-old girlfriend of his partner, Stephen "The Rifleman" Flemmi, and Deborah Hussey, also 26, the daughter of Flemmi's common-law wife. In both cases, Bulger insisted on pulling out the women's teeth so they would be difficult to identify, Flemmi testified.

During a search of his Santa Monica apartment, agents found over $800,000 in cash and more than 30 guns, many hidden in holes in the walls. A property manager at the building said Bulger and Greig, who used the names Charles and Carol Gasko, had lived there for 15 years and always paid the rent-controlled rate of $1,145 a month in cash.

They were caught days after the FBI began a new publicity campaign focusing on Greig. The daytime TV announcements showed photos of Greig and noted that she was known to frequent beauty salons and have her teeth cleaned once a month.

A woman from Iceland who knew Bulger and Greig in Santa Monica saw a report on CNN about the latest publicity campaign and called in the tip that led agents to them. The Boston Globe identified the tipster as a former Miss Iceland, a former actress who starred in Noxzema shaving cream commercials in the 1970s.

Greig is still serving her sentence at a federal prison in Minnesota.

[NATL-NECN] Whitey Bulger's Belongings Auctioned for Over $100,000

Clothing, jewelry, books and other possessions once owned by Bulger and Greig went up for auction at the Boston Convention Center and online in 2016, with the money going to Bulger's victims' families.

Bulger, a physical fitness buff, had been taken to a Boston hospital from his jail cell at least three times, complaining of chest pains, since being brought back to Boston to stand trial.