The F.B.I. said Monday that it believes it knows the identity of the thieves who stole 13 paintings 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.