A little after midnight on March 18th, 1990, two men dressed as police officers knocked on the door of Boston's Isabella Stewart Gardner Museum, and were let in by an overnight worker there.  What happened next has been a mystery for more than 23 years.

But what authorities and the FBI knew was that more than $500 million worth of artwork had been stolen, 13 pieces of artwork in 81 minutes.

Over the years, the FBI chased various leads across the globe and they now believe that they know who pulled off the heist.  The agency now thinks that the thieves were part of a criminal organization  based in New England and the mid-Atlantic states.

They think that the artwork was taken to Connecticut and then the  Philadelphia region in the years after the theft, and finally offered for sale in the Philadelphia area about a  decade ago. After that, the FBI lost the lead.  They decline to offer the names of suspects that they are still investigating.

FBI.Gov picture - Manet, Chez Tortoni, 1878–1880

Since the heist, empty frames have hung on the walls of the museum, reminding people of the enormous loss that the museum had incurred.

The FBI has now begun a publicity campaign to recover the stolen artwork. There is a $5 million dollar reward for its return.

The FBI's Richard DesLauriers said authorities believe someone not involved in the theft has seen the artwork without realizing it is stolen. "It's  likely over the years that someone, a friend, neighbor or relative, has seen the art hanging on a wall, placed above a mantle or stored in  an attic. We want that person to call us," DesLauriers said.

“Twenty-three years since the robbery. That’s far too long. It’s time for these paintings to come home.” - Anthony Amore, Isabella Stewart Gardner Museum