BOSTON (CBS) – Tips are pouring in from the FBI’s latest push to find $500 million worth of art stolen from the Isabella Stewart Gardner Museum 23 years ago.

On March 18, 1990, two men disguised as police officers tied up the security guards and stole 13 objects, including rare paintings by Rembrandt, Degas, and Vermeer. It was the largest property crime in U.S. history.

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The FBI and museum say people from all over the country are responding to a public awareness campaign launched Monday, including a $5 million reward for the return of the art.

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Twenty-five billboards have gone up in the Philadelphia area, asking for information about the stolen art and a new public service announcement has been released.

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Citing a tip this week, the FBI re-dedicated resources to solving the case. Investigators announced Monday they now believe they knew who did it, but the suspects have not been named.