Two decades after a pair of thieves dressed as Boston police officers pulled off the biggest art heist in history, the FBI is trying to stir up new leads with two billboards on Boston-area freeways that promise a $5 million reward in the case.

The FBI has also resubmitted DNA samples for updated testing, the Associated Press reports.

The thieves made off with 13 artworks worth $500 million from Boston's elegant Isabella Stewart Gardner Museum, including three Rembrandts, a Vermeer, a Manet and five sketches by Degas.

The two men pulled off the robbery on March 18, 1990, by posing as Boston police officers then overpowering two guards in the museum, which had only minimum security.

The two digital billboards, donated by Clear Channel Outdoor, will be up for about four weeks along Interstate highways in Stoneham and Lawrence, The Boston Globe says.

Federal authorities are offering immunity from prosecution and promising confidentiality in exchange for the artwork. The Gardner museum is offering the $5 million reward.

The billboards ask anyone with information to contact the FBI or go to the museum's website here.

(Posted by Doug Stanglin)