BOSTON (CBS) – The Isabella Stewart Gardner Museum has announced that it is doubling its reward for information leading to the return of 13 works of art that were stolen in 1990.

The museum’s Board of Trustees announced Tuesday that it has increased the reward from $5 million to $10 million. The reward is “available immediately” but expires at midnight on December 31, 2017.

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“We encourage anyone with information to contact the Museum directly, and we guarantee complete confidentiality,” said Anthony Amore, the Museum’s Security Director. “This offer is a sign that our investigation remains active. Our hope is that anyone with knowledge that might further our work will come forward.”

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Empty frame at Isabella Stewart Gardner Museum (CBS)

On March 18, 1990, two men disguised as police officers tied up the security guards and stole 13 pieces of art, including rare paintings by Rembrandt, Degas, and Vermeer from the museum in Boston.

CHECK: Stolen Art Catalogued

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The combined value of the art is estimate at $500 million. It remains the largest art heist in history.