For the last 23 years, 13 empty frames have hung on the walls of Boston’s Isabella Stewart Gardner Museum. The frames represent the 13 paintings that were stolen in the biggest art theft in U.S. history, when two men disguised as police officers swiped approximately $300 million worth of art from the museum. The FBI identified the thieves earlier this year, but the paintings—by artists including Degas, Rembrandt and Vermeer—remain at large.

Besides inspiring a documentary (2006’s Stolen), the heist, and in particular the empty frames, spurred an exhibition by artist Sophie Calle. Last Seen includes 14 photographic and text-based works that Calle created both in 1991, shortly after the theft, and more recent works from 2012. The artist reflects on absence and memory by showing museum staff members and visitors posing in front of the empty frames. Find out more at gardnermuseum.org.