Police closed off entries and exits at the Senate's Dirksen Office Building on Monday and announced that people should avoid the building's basement.

Capitol Police Sgt. Kimberly Schneider said an alarm went off about 4:50 p.m. indicating a suspicious substance near a basement post office facility.

The detector signaled the possible presence of a nerve agent, but initial field tests were negative, a law enforcement official said on condition of anonymity because authorities were still investigating.

Fire trucks and ambulances gathered along Constitution Avenue and First Street between the Capitol grounds and the office building. Decontamination tents were erected at the site and three fully outfitted hazardous materials specialists were seen going through them.