BIRMINGHAM, Ala. (AP) — A federal courthouse in Birmingham is closed as authorities try to determine the source of a white powder that was found in the building over the weekend.

The Robert Vance Federal Building is scheduled to reopen Wednesday.

An employee catching up on work discovered an envelope containing white powder on Sunday. Initial tests for anthrax were negative, but officials say five people are being treated with antibiotics after potentially being exposed to the substance.

The building will be closed on Monday and Tuesday as a precaution. It houses the U.S. Bankruptcy Court for Birmingham.