A look into the largest unsolved art heist in history: the theft of 13 irreplaceable artworks from the Isabella Stewart Gardner Museum in Boston. "Last Seen" begins Sept. 17.

In 1990, two thieves stole 13 irreplaceable artworks from the Isabella Stewart Gardner Museum. We take a closer look at what happened that night.

On the night of the heist, security guard Rick Abath made the critical mistake of letting the thieves into the museum. In this episode, we ask if it was indeed a mistake.

Was the heist planned in the belly of Boston's criminal underworld operating out of a Dorchester auto body shop?

Were George Reissfelder and David Turner involved in the Gardner heist?

We trace the art's possible path from Boston to Connecticut to Philadelphia.

This is a story about how to plot an art recovery, and then blow it entirely.

Was the world's greatest art thief the inspiration, or actually the mastermind, of the Gardner heist?