The F.B.I. said Monday that it believes it knows the identity of the thieves who stole 13 works of art 23 years ago from the Isabella Stewart Gardner Museum in Boston, one of the most infamous art heists in history.
Officials from the F.B.I. said they believed that the paintings were moved through Connecticut and the Philadelphia area perhaps a decade ago by a criminal organization. They declined to reveal any more about the identity of the thieves, saying the investigation is continuing.
The F.B.I. is establishing a Web site, www.FBI.gov/gardner, as part of a publicity campaign to alert the public. That campaign includes billboards to be placed in Connecticut and Philadelphia, with reproductions of the paintings in hopes of prompting anyone with information to step forward.
The museum is still offering a $5 million reward for information that leads to the recovery of the art work in good condition. It is valued at up to $500 million.
The bureau undertook a similar publicity effort a few years ago in seeking information about James (Whitey) Bulger, the Boston mobster who had been living on the lam for more than a decade. The campaign led to his arrest in California.
The announcement on Monday was intended to alert potential informants beyond the Boston area, which has been obsessed with the crime since it occurred. "We are expanding the aperture of awareness," Richard DesLauriers, the special agent in charge of the F.B.I.'s Boston field office said at a news conference. He emphasized that the office does not know where the paintings are now.
The theft occurred at 1:20 a.m. on March 18, 1990. A young night watchman let two men disguised as police officers into the museum after they rang the intercom at the service entrance and claimed they were responding to a disturbance. The thieves subdued the guard and his lone overnight colleague and locked them in the basement bound in handcuffs and duct tape.
The two men removed a total of 13 items in 81 minutes. Included were two large Rembrandt oil paintings that were cut from their frames; single works by Vermeer, Manet and Govaert Flinck; five Degas sketches, and three other items, among them a small etching by Rembrandt.
The robbery ranks as the single biggest museum theft in history in terms of the potential sales value of the missing works. The F.B.I. puts the figure at $300 million, though others put it at $500 million.
Over the last 23 years investigators have questioned the relatives and associates of about a dozen Boston-area criminals. Those individuals were part of a loose confederation of New England underworld figures, some with Mafia ties and a few of whom have died.
According to federal court records, F.B.I. affidavits and interviews with lawyers familiar with grand jury proceedings, a lynchpin is Robert Guarente, a Mafia figure who died in 2004 at age 65. Officials have searched his former residences, looked into his past movements and studied the activities of many of his associates.
Katharine Q. Seelye reported from New York, and Tom Mashberg from Boston.
Correction: March 18, 2013, Monday
This article has been revised to reflect the following correction: An earlier version of this article misstated the type of artwork stolen from the Isabella Stewart Gardner Museum in Boston. It included paintings, a finial and an ancient bronze beaker, not just paintings.
This article originally appeared in The New York Times.