On March 18, 1990, two men dressed as police officers pulled off the biggest art heist in American history.

The Isabella Stewart Gardener Museum in Boston houses priceless works of art worth hundreds of millions of dollars - but the collection used to be worth a whole lot more. 

The thieves of the 1990 heist walked in unnoticed, sliced several painting right out of their frames, and grabbed a few artifacts as well. Overall, what they took is valued at nearly half a billion dollars. Investigators suspect it was the work of Boston organized crime, and some well-known underworld names were brought forward - but the case remains unsolved to this day. 

Here are some of the pieces stolen, some of the prime suspects, and the empty frames that still hang in the museum to this day, waiting for the artwork they house to return.