Just after midnight on March 18, 1990, two thieves dressed as police officers entered Boston's Isabella Stewart Gardner Museum and made off with 13 paintings worth a sum of half a billion dollars. For years the mystery has intrigued the art world as the whereabouts of the paintings remains unknown — even as the picture frames remain hanging on the walls of the museum as ghostly reminders of what was taken.

In an effort to expedite the recovery of the art, the museum offered a stunning reward of $10 million earlier this year to anyone with information leading to the recovery of the property, which includes art by Rembrandt, Vermeer, and Degas, The New York Times reports. But as of Jan. 1, that reward will drop back to $5 million. As the museum's director of security, Anthony Amore, put it: "It's conceivable that some criminal organization or people might be wishy-washy about the $5 million. But it's not conceivable that they're feeling the same way about the $10 million."

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There have been a few leads over the years, with the FBI claiming that they know the identity of the original thieves, who have allegedly since died. But any effort to discover who possesses the art now has been fruitless. "The investigation has had many twists and turns, promising leads and dead ends," FBI spokeswoman Kristen M. Setera told the Times. "There is no part of the world the FBI has not scoured following up on credible leads." Read more about the theft and the massive reward at The New York Times. Jeva Lange