A cast of shady characters

The early investigation led authorities down several paths, from the Boston mafia to the IRA.

Some theories suggest that a robbery of this magnitude couldn't have been pulled off without the "blessing" of local organized crime figures, suggesting James "Whitey" Bulger, head of Boston's Irish mob, has some knowledge of the robbery or the paintings' whereabouts.

James "Whitey" Bulger

Bulger, who has been on the run since 1995 after a disgraced FBI agent tipped him off that he was about to be indicted for racketeering and extortion, had acted as a government informant for two decades. At the same time, he rose through the ranks of Boston's Winter Hill Gang. He was added to the FBI's 10 Most Wanted list in 2000 and there is a $1 million reward for information leading to his capture.

The IRA was also a focus of the initial investigation.

The organization itself (which is dedicated to ending British rule in Northern Ireland and was considered a terrorist organization by the U.S. State Department until two years ago), has a history of involvement with "art theft on a big scale," according to art investigator Harold Smith, who in retirement has made the Gardner heist his personal crusade.

In 1974 an IRA gang robbed, Russborough House, a private estate near Dublin that houses major works of art, of 19 paintings, including a Vermeer, a Goya and two Gainsboroughs. The works, which were pried from their frames with screwdrivers, were later used in an attempt to barter for the transfer of four of the group's imprisoned members.

The art, however, was recovered less than two days later. Since that heist, the estate has been targeted for art theft on three other occasions.

Another early suspect in the investigation — by his own admission — was antiques dealer and Scotland Yard informant Michael van Rijn.

The main entrance of the Isabella Stewart Gardner Museum

Van Rijn, who was living in New York City at the time of the robbery, denies involvement in the heist, but claimed in an article in Atlantic Monthly magazine to have information regarding who did, and possibly on the paintings' whereabouts. Saying he feared for his life, van Rijn refused to divulge anything and went into hiding in London shortly after the article appeared in November 2001.

The investigation has revolved mainly around two convicted criminals from the Boston area — Myles Connor, a notorious art thief, and his associate, William Youngworth, a career criminal.

Though in a Rhode Island prison serving a 15-year sentence for trafficking stolen antiques in Rhode Island at the time of the 1990 robbery, Connor surfaced early on in the investigation as a possible mastermind of the heist. From behind bars and after he was released on parole, Connor continued to lead the feds down several winding paths — and remains a significant figure in the case.