The FBI has issued a $25,000 reward after seven of Campbell’s Soup Cans prints by Andy Warhol—worth an estimated $500,000—were stolen from the Springfield Art Museum in Missouri.

It is thought that the heist took place after 5:30 pm, when the museum, which does not have night security, closed its doors to the public for the day. No one noticed the prints were missing until the museum opened at 8:45 am the following day.

The thieves are thought to have broken into the museum when it was closing and then made off with the prints during the night. Because they only stole seven of the 10 prints in the soup cans series, the value of what they stole has been severely lessened.

The print series in question, Campbell’s Soup 1, was made in 1968, after the success of Warhol’s groundbreaking first series of paintings featuring the now iconic cans, which he made in 1962. The print series has been part of the Springfield Art Museum’s collection since 1985.

Lisa Cox, a Springfield Police spokeswoman, told the New York Times that the prints were the pride of Springfield. “They were one of those kinds of ‘claim to fame’ types of pieces,” she said.

The prints, measuring three feet tall and two feet wide, were rarely put on display due to their fragility. It is thought that only 50 undamaged or complete sets of 10 of these prints remain in existence.

Because the works are so well known, it will be extremely difficult for the thieves to pass them off in a legal sale or auction, which will surely force them onto the black market.

“Whoever they sell it to would need to know that they’re stolen,”Ron Rivlin, owner of Revolver Gallery, which specializes in Warhol works, told the NYT.

The FBI’s art crime unit is currently investigating the crime in co-operation with the Springfield Police Department, but they have no leads as to who committed the crime yet.