A silkscreen used to create Andy's Warhol's famous "Flowers" was among 19 modern and contemporary artworks stolen from a Detroit collector.
The owner had temporarily stored his art at his business.
"It could be somebody that could have had prior knowledge of where this art was going to be. Or it could've been a crime of opportunity where somebody happened to be inside the building and had seen it," said a FBI spokesman.
A $5,000 reward is offered for information. The collection is worth in the multimillions and includes drawings, prints and paintings by artists such as Larry Rivers, Francesco Clemente, Philip Taaffe, Peter Schuyff and Joseph Beuys.
"There's a strong likelihood that this artwork has been already taken either out of state or possibly even out of the country," the FBI spokesman said. "So we're putting out the message to anyone in the art community, anyone who's shopping online and looking at art, anyone who's involved in the pawn shop business, that these could all be avenues that someone who's stolen this art could pursue in order to try to make a profit."