Police closed off entries and exits at the Senate’s Dirksen Office Building for several hours Monday after something set off an alarm for the presence of nerve agents.

Sgt. Kimberly Schneider, a Capitol Police spokeswoman, said an alarm went off about 4:50 p.m. indicating a suspicious substance near a basement post office facility. An all-clear was declared about 8:15 p.m., after tests proved negative.

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

Schneider said, “The fact that we haven’t had any positive hits on any nerve agents is good.” It was not known what set off the sensors, she said, and that will be subject to further investigation.