Over 200 people, including eight Senators, were evacuated from the Russell Senate Office Building Wednesday night after a sensor in the building’s attic indicated the presence of a nerve agent, but it was later determined to be a false alarm.

Capitol Police spokeswoman Sgt. Kimberly Schneider said that “something in the air” triggered the alarm, and she could not immediately tell what it was, but that it was possibly a cleaning solvent.

The alarm sounded at approximately 7 p.m. and evacuees were led to the underground parking garage next to the building, according to Senate aides who were evacuated. The all clear was given around 9:30 p.m.

Schneider said no one received treatment because none of the evacuees reported having any symptoms of nerve agent exposure — not even a runny nose.

Police echoed that no one has shown symptoms of exposure, leading one security expert to suspect a false alarm before it was even announced. — CNN

See also: Associated Press

It seems those sensors are way too sensitive, or not calibrated properly, or perhaps their design doesn’t allow them to distinguish between nerve gas and cleaning solvent. This isn’t the first time sensors in D.C. have acted strangely. In September, sensors on the Mall in D.C. detected the presence of tularemia bacteria.