BOSTON—A federal grand jury will hear evidence in the unresolved 1990 art heist at the Isabella Stewart Gardner Museum, according to a former employee who worked at the museum at the time, the Boston Globe reports.

The former worker, who asked not to be identified, said two FBI agents questioned him about his recollection of the theft several days ago and handed him a subpoena to testify before the grand jury in Boston tomorrow. According to the former worker, the agents said they were pursuing whether three individuals committed the crime—instead of two, as has long been thought.

The theft happened In the early morning of March 18, 1990, when, according to press reports, two men dressed in Boston police uniforms stole 13 pieces of artwork, including three Rembrandts and a Vermeer, estimated to be worth between $300 million and $500 million total.