Via WJLA:

The U.S. Capitol Police are investigating a possible hazmat situation near the National Mall.

D.C. Fire officials confirm they have been called to assist police with the situation, which they say might include a mentally unfit subject.

The investigation is at 3rd St and Madison Dr NW. Officials say 3rd St SW between Independence Ave SW and Constitution Ave NW is closed an they ask the public to avoid the area.

UPDATE: Robot approaching vehicle.

UPDATE: Video from the scene courtesy of VOA News: