Notorious mob boss James "Whitey" Bulger was due in court on Thursday after his arrest in Los Angeles ended 16 years on the run for a string of murders which inspired gangster film The Departed.
The Federal Bureau of Investigation (FBI) said a recent publicity campaign had generated a tip that led agents to arrest Bulger, 81, and his longtime companion Catherine Greig, 60, "without incident" late on Wednesday.
Bulger was spotted in London in 2002 and in California in 2000 and 2005 but had evaded arrest even since he fled shortly before his federal racketeering indictment in January 1995.
It later emerged that he had been a long time FBI informant, fueling suspicion about the agency's fruitless efforts to find the fugitive, according to the Boston Globe newspaper.
The head of the FBI in Boston, Special Agent Richard Deslauriers, said "a variety of weapons, guns, and a very substantial amount of cash" were found in the apartment where the pair was detained in Santa Monica.
Prosecutors hailed the arrest.
"My reaction was I was extremely grateful ... we've gone a long way to bringing justice to the victims and their families," US Attorney Carmen Ortiz told a press conference in Boston.
"Now we have the ability to at least bring him to face these charges," she added.
Bulger, who has been indicted for 19 murders, was the inspiration for Jack Nicholson's character in The Departed, the gritty 2006 crime film directed by Martin Scorsese and also starring Leonardo DiCaprio and Matt Damon.
The FBI began airing public service announcements on Tuesday that were focused on Greig -- a former dental hygienist said to frequent beauty salons -- whom law enforcement agents believed would lead them to Bulger.
It had offered a $100,000 reward for information on her whereabouts and a $2 million reward for information leading to the arrest of Bulger -- the largest reward the FBI has ever offered for a Top Ten domestic fugitive.
The FBI said on its Ten Most Wanted website that Bulger had been sought for a string of murders from the early 1970s to the mid-1980s "in connection with his leadership of an organized crime group."
It said his gang "allegedly controlled extortion, drug deals, and other illegal activities in the Boston, Massachusetts, area."
Bulger had used a number of aliases during years on the run, the FBI said before the arrest, adding that he "has a violent temper and is known to carry a knife at all times."
International police body Interpol praised the arrest, saying it showed nobody could evade justice forever.
"The arrest of James Bulger and his girlfriend after so many years on the run is testimony to the dedication and tenacity of the FBI officers involved," said Stefano Carvelli of Interpol's Fugitive Investigative Support unit.
"It is once again a clear signal for all fugitives everywhere, that no matter how much time has passed, once they are wanted they will remain wanted until brought to justice," he added in a statement in Lyon, France.
The FBI said both Bulger and Greig, who is accused of harboring a known fugitive, would appear in court in Los Angeles at 2:00pm on Thursday.