The FBI is celebrating the capture of an octogenarian mobster who has been on the lam for the past 16 years.

James "Whitey" Bulger was captured late Wednesday in Santa Monica, Calif., at an apartment building where he had been living with his long-time girlfriend.

FBI agents swarmed the three-storey building known as The Princess Eugenia, and later lured the 81-year-old Bulger and his 60-year-old girlfriend Catherine Greig out using an undisclosed ruse.

Both Greig and Bulger are expected to appear in a Los Angeles court on Thursday.

The couple were caught only days after the government launched a new publicity campaign to locate the former Boston mob boss. Inside Bulger's residence, FBI agents said they found $800,000 in cash, several pieces of false identification and more than 30 firearms.

Barbara Gluck had been living on the same floor as Bulger and Greig and did not know who they were until they were captured.

Gluck said she occasionally spoke to Greig when she saw her in the Santa Monica building. But Bulger would always tell his girlfriend to "stop talking" to other people.

"He was nasty," said Gluck, who described Bulger as having an apparent "rage issue."

Bulger had been on the run since January 1995, when he learned that the FBI intended to indict him.

Wanted for his alleged role in 19 murders, Bulger had been one of the top-10 most wanted men in the United States up until his capture. There was a long-standing $2-million reward in place for information leading to his arrest.

During his years on the run, the FBI was told about reported sightings of Bulger in cities across North America and in Europe.

In one such case last year, it was believed that Bulger may have visited bookstores in British Columbia. The FBI said he was known to be "an avid reader" who often visited libraries and historic sites.

When Bulger was young, he had a shock of bright platinum hair, which led to his nickname "Whitey." But he also known by many other names, including Thomas F. Baxter, Mark Shapeton, Tom Harris, Tom Marshall, Ernest E. Beaudreau, Harold W. Evers and Robert William Hanson.

Bulger was using the name Charles Gasko when he was caught on Wednesday.

Prior to going undergound, Bulger and Stephen "The Rifleman" Flemmi led the so-called Winter Hill Gang -- a largely Irish-American mob that was involved in loan-sharking, gambling and drug rackets in the Boston area.

Five years after Bulger fled, U.S. Attorney Donald K. Stern said Flemmi and the fugitive were "responsible for a reign of intimidation and murder that spanned 25 years."

In addition to the murder charges, Bulger also faces federal charges for conspiracy to commit murder, narcotics distribution, extortion and money laundering.

Greig is charged with harbouring a fugitive.

Like Bulger, the FBI had a reward in place for information leading to Greig's arrest. But her reward was pegged at $100,000 -- only five per cent of the value of the $2-million reward linked to Bulger's capture.

With files from The Associated Press