New Clues in Art Heist Mystery

March 11 -- Two Polaroid photos in a plain envelope with no return address arrived by mail at the ABCNEWS office in New York. But despite their source's anonymity, if authentic, they could provide clues into a heist the FBI calls the largest art theft in American history.

They were photos of what appeared to be a missing Rembrandt masterpiece, stolen on March 18, 1990, from Boston's Isabella Stewart Gardner Museum, a four-story museum packed with priceless masterpieces from the collection of socialite Isabella Stewart Gardner.

The alleged Rembrandt was placed atop of a Boston Globe front page to show the date and next to a tape measure to show it to be the size of the rare self-portrait, which isn't much bigger than a postage stamp — but is priceless.

The photo could be an elaborate hoax. But, if authentic, it could prove that the Rembrandt still exists and could be an important clue in a case that for 14 years, the FBI has been unable to break.

After a seven-year investigation into this case, ABCNEWS has learned of a secret 1997 meeting between a one-time member of an art theft ring and museum officials, and that a new deal with the FBI may be in the works.

The 1990 break-in led to an international search for the 12 masterpieces, including works by Rembrandt, Vermeer, Degas and others, which are likely worth as much as a half-billion dollars, according to the museum and the FBI.

View images of the missing masterpieces

"To have art of that quality … taken and never to be seen, never to be appreciated by anybody again, is a devastating impact on the city and frankly, on the art world," said Donald Stern, the former U.S. attorney for Massachusetts.

Despite a $5 million reward, the art is still missing and law enforcement is no closer to making an arrest. When asked if he thought the investigation has been, so far, a failure, Stern responded, "I guess you have to say that."