Rembrandt’s “The Storm on the Sea of Galilee,’’ his only seascape, has been plastered on Massachusetts billboards along two interstates in an effort by the FBI to publicize the Gardner Museum theft two decades after it transpired.

In 1990, two men disguised as Boston police officers talked their way into the museum, bound guards, and stole artwork valued at $500 million, including three Rembrandts, a Vermeer, a Manet, a Flinck, five sketches by Degas, the eagle finial from atop a Napoleonic flag, and a Chinese beaker.

For the art's safe return, the Gardner is offering a $5 million reward and the FBI is offering immunity from prosecution.