(CNN) In a 3-foot grave near a river not far from Boston, Paul McGonagle's body lay decaying, undisturbed until his remains were found nearly 20 years ago. Parts of a pelvic bone. A fractured skull with decomposed brain matter. A ring with a part of a decomposed finger bone.

McGonagle was the leader of a gang that was a rival to reputed Irish mobster boss James "Whitey" Bulger's Winter Hill Gang. Bulger was suspected of being involved in McGonagle's death in 1974, and in 18 other mob-related killings between 1973 and 1985.

Whenever Bulger drove by the shallow grave, he would say "Drink up, Paulie," a witness testified at Bulger's trial in 2013. A federal jury convicted Bulger of 31 counts that year, including racketeering and extortion, and found him culpable in 11 killings , including McGonagle's death.

Bulger, leader of the South Boston gang, was the ruthless kingpin of a criminal empire that a federal judge said committed "unfathomable" acts. His infamous reign would be the inspiration for Jack Nicholson's character in Martin Scorsese's film "The Departed." Bulger evaded police for 16 years before his capture in 2011.

"The guy is a sociopathic killer," Tom Foley, the former Massachusetts State police head who spent years trying to capture Bulger, told CNN in 2011. "He loved that type of life. He's one of the hardest and cruelest individuals that operated in the Boston area. He's a bad, bad, bad guy."

Bulger, a longtime FBI informant, was killed in the US Penitentiary Hazelton in Bruceton Mills. He was found unresponsive at 8:20 a.m., according to statement from the Federal Bureau of Prisons.

He was pronounced dead by the Preston County medical examiner after life saving measures failed. No other inmates or staff were injured, the prisons bureau said.

The FBI is investigating Bulger's death, which occurred a day after he was transferred to the West Virginia facility, the prison bureau said.

'It's a happy day'

A statement Tuesday from Andrew Lelling, the US Attorney for Massachusetts, was brief. It made no mention of Bulger other than he had died.

"We received word this morning about the death of James "Whitey" Bulger. Our thoughts are with his victims and their families," the statement said.

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The brother of a woman Bulger was accused of killing in 1981 told CNN, "It's a happy day."

Steven Davis' 26-year-old sister, Debra Davis, was the girlfriend of Bulger partner Steve "The Rifleman" Flemmi. Flemmi testified that he lured Davis to a house where Bulger strangled her. Flemmi testified he "inadvertently blurted out" to Davis that he and Bulger were FBI informants, and that Bulger said several times "he wanted to kill" Davis because she knew about the relationship with the FBI.

The defense presented testimony from former hitman John Martorano, who admitted he "accidentally strangled" Davis. Martorano served 12 years in prison, but was released in 2007 in exchange for testifying against Bulger.

The jury had "no finding" in Davis' death. Bulger "didn't have the right to live as long as he did," her brother said.

The end of a '16-year honeymoon'

In 1995, Bulger skipped town ahead of a pending indictment, allegedly on a tip from a rogue FBI agent.

The mob boss, who took Osama bin Laden's place at the top of the FBI's Top 10 Most Wanted list, hid in plain sight in Santa Monica, California. He and his girlfriend, Catherine Elizabeth Greig, lived blocks from the beach under the aliases of Charlie and Carol Gasko. The fake name was even on the door bell list.

The couple dined some nights at a white-tablecloth establishment that drew Hollywood producers, sitting in a corner table in the patio at the back of the restaurant, a manager said. They were generally polite, too. Bulger often wore a hat, with the brim pulled down, neighbor Catalina Schlank told CNN in 2011.

"They were kind of secretive. Even on the phone -- you couldn't call them," she said. "Sometimes, I wanted to give them a tip of stuff on sale."

A tip led the authorities to him.

In 2011, the FBI lured a likely unsuspecting Bulger out of his seaside apartment. Bulger had received a phone call and was told that his lockbox had been broken into in the basement parking lot area of his building. The FBI arrested Bulger when he went to check it out, a source told CNN.

Bulger would refer to his years as a fugitive with Greig as a "16-year honeymoon."

to eight years in federal prison for identity fraud and helping Bulger avoid capture. In 2012, Greig was sentenced to eight years in federal prison for identity fraud and helping Bulger avoid capture.

Photos: Who's who in the James 'Whitey' Bulger case Photos: Who's who in the James 'Whitey' Bulger case James "Whitey" Bulger, the former head of Boston's Winter Hill Gang, evaded police for 16 years before his 2011 arrest with girlfriend Catherine Greig in Santa Monica, California. After a lengthy trial, Bulger, seen here in his booking photo from June 23, 2011, was found guilty on 31 of 32 counts -- including involvement in 11 murders. On November 14, 2013, Bulger was given two life sentences plus five years. Here's a look at some of the people tied to Bulger's life of crime: Hide Caption 1 of 23 Photos: Who's who in the James 'Whitey' Bulger case James "Whitey" Bulger, seen here in a 1984 FBI photo, spent nine years in federal prison before he climbed the ranks of the Winter Hill Gang -- the preeminent Irish-American crime syndicate in the Boston area -- in the early 1970s. Hide Caption 2 of 23 Photos: Who's who in the James 'Whitey' Bulger case According to prosecutors, Bulger's crew learned that a bookie named Richard Castucci was cooperating with the government, and John Martorano was sent to kill him. Castucci was shot in the head in December 1976 and stuffed in a sleeping bag in the back of his car. Hide Caption 3 of 23 Photos: Who's who in the James 'Whitey' Bulger case Steve "The Rifleman" Flemmi, left, and bookie Dick O'Brien in one of several surveillance photographs entered into evidence in the Bulger trial. Flemmi, Bulger's partner, would meet O'Brien to collect thousands of dollars in "rent" every month. Hide Caption 4 of 23 Photos: Who's who in the James 'Whitey' Bulger case Bulger is accused of murdering Flemmi's stepdaughter, Deborah Hussey, in 1985 because she became a liability. Hide Caption 5 of 23 Photos: Who's who in the James 'Whitey' Bulger case Flemmi met Debra Davis at a jewelry store, and the couple dated for more than seven years. In 1981, Bulger is said to have killed Davis because she knew that Flemmi was an informant. Hide Caption 6 of 23 Photos: Who's who in the James 'Whitey' Bulger case Dr. Ann Marie Mires, a Massachusetts state forensic anthropologist, was brought in to Bulger's trial to show photos of his alleged victims, including Debra Davis. Because Davis' body was put into bags, almost all of her remains were recovered. Even some of her hair was preserved. Hide Caption 7 of 23 Photos: Who's who in the James 'Whitey' Bulger case Mug shots of Bulger in 1953, about a year after his honorable discharge from the U.S. Air Force. Hide Caption 8 of 23 Photos: Who's who in the James 'Whitey' Bulger case Bulger was the godfather to John Martorano's first son. Martorano has admitted to 20 killings as part of Boston's Winter Hill Gang and was the government's star witness against Bulger. Hide Caption 9 of 23 Photos: Who's who in the James 'Whitey' Bulger case In 2008, John Martorano, pictured here, testified against former FBI agent John Connolly, who was accused of leaking sensitive information about former gambling executive John Callahan. Martorano testified that he shot his friend Callahan on Bulger's orders in 1982. Hide Caption 10 of 23 Photos: Who's who in the James 'Whitey' Bulger case John Callahan was an organized crime associate of the Winter Hill Gang and former president of World Jai Alai. Prosecutors allege Bulger ordered a hit on Callahan after he learned he would be cooperating with the feds on the high-profile murder of an Oklahoma businessman, Roger Wheeler. Hide Caption 11 of 23 Photos: Who's who in the James 'Whitey' Bulger case Bulger is accused in the slaying of Wheeler, who was gunned down outside a country club in Oklahoma in 1981. Hide Caption 12 of 23 Photos: Who's who in the James 'Whitey' Bulger case Joe Notorangeli was gunned down by the Winter Hill gang in 1973, according to Martorano. Hide Caption 13 of 23 Photos: Who's who in the James 'Whitey' Bulger case John Connolly was convicted of second-degree murder in the slaying of Callahan and received a 40-year sentence in 2009. Connolly is appealing. Hide Caption 14 of 23 Photos: Who's who in the James 'Whitey' Bulger case Former FBI supervisor John Morris testified at Bulger's trial that he provided information to Bulger in exchange for money and gifts. Here, Morris testifies during the John Connolly murder trial in Miami in 2008. Hide Caption 15 of 23 Photos: Who's who in the James 'Whitey' Bulger case Bulger's girlfriend, Catherine Greig, was sentenced to eight years in federal prison in 2012 for identity fraud and helping the reputed mob boss avoid capture for 16 years. Hide Caption 16 of 23 Photos: Who's who in the James 'Whitey' Bulger case J.W. Carney, Bulger's defense attorney, arrives at the U.S. Federal Courthouse for the start of Bulger's trial in Boston on Wednesday, June 12, 2013. Hide Caption 17 of 23 Photos: Who's who in the James 'Whitey' Bulger case This undated surveillance photo released by the U.S. Attorney's Office in Boston shows Bulger, left, with his former right-hand man, Kevin Weeks. Weeks took the witness stand at Bulger's racketeering trial and described a double slaying, multiple extortions and drug dealing. Hide Caption 18 of 23 Photos: Who's who in the James 'Whitey' Bulger case Bulger and Kevin Weeks walk around Castle Island on Boston Harbor. Hide Caption 19 of 23 Photos: Who's who in the James 'Whitey' Bulger case Kevin Weeks leaves the courthouse in July 2013 after testifying in graphic detail about how Bulger killed Arthur "Bucky" Barrett, Joey McIntyre and Deborah Hussey. Hide Caption 20 of 23 Photos: Who's who in the James 'Whitey' Bulger case The remains of Thomas King, former member of the Winter Hill Gang, were found in late 2000 and included these driving gloves, a bulletproof vest, a navy suit, and a claddagh ring. Martorano, one of Bulger's hitmen, testified that he himself had shot King in the back of the head. Hide Caption 21 of 23 Photos: Who's who in the James 'Whitey' Bulger case Stephen Rakes, 59, was scheduled to be a witness for the prosecution before he was dropped from the list. His body was found July 17, 2013, in Lincoln, Massachusetts, west of Boston. Rakes' business associate has been charged with his murder. Authorities said Rakes' killing was unrelated to the Bulger case. Hide Caption 22 of 23 Photos: Who's who in the James 'Whitey' Bulger case Trying to show a softer, lighter side of Bulger, his defense lawyers released numerous photos of their client during the 2013 trial. The tactic didn't work: Bulger will spend the rest of his life in prison. Hide Caption 23 of 23

During Bulger's trial, gruesome details emerged about the deaths he was accused of being involved in.

In the summer of 1983, Bulger organized a meeting with Barrett under the guise that he had stolen diamonds he wanted to get rid of. When Barrett showed up at the house, Weeks, Bulger and Flemmi were waiting.

They chained Barrett to a chair. For hours, they grilled him about a rival gang and local drug dealings, Weeks testified.

Barrett tried to buy his way to out, telling Bulger about his stash of $40,000.

Weeks said Bulger left the house to pick up the money while he and another associate watched Barrett. When Bulger returned, he told Barrett to walk down to the basement. Then Bulger put a gun to the back of Barrett's head and pulled the trigger, according to Weeks.

"Nothing happened," Weeks said.

Bulger realized the gun's safety was on. He removed the safety and shot Barrett, Weeks said.

'It's just sad that it took so long'

During his sentencing, Bulger did not make eye contact with the relatives of the people he killed, nor those slain by his accomplices in the Winter Hill Gang.

He was a convicted murderer in his 80s. He was likely going to die in prison.

Carmen Ortiz, the former US Attorney for Massachusetts who oversaw Bulger's prosecution, said she hopes Bulger's notoriety didn't take away the focus from his victims. Ortiz says she hopes his death "is the end of a very sad chapter in Boston's history, during which this man caused so much harm to many through his brutal crimes."

"He had it coming to him and it's just sad that it took so long," Steven Davis said.