Several U.S. Senators and their staffers were evacuated from an office building near the U.S. Capitol Wednesday evening, after sensors detected traces of a chemical or biological nerve agent.

A U.S. Capitol Police spokeswoman said an alarm went off in the attic of the Russell Senate Office Building. She said some 200 people, including about a dozen Senators, evacuated to the building's parking garage to be quarantined. Police were quoted as say none is exhibiting symptoms of contamination at the present time.

Initial test results were reported to have come back positive for nerve agent, but a second came back negative. Specimens have been sent to a testing facility in Washington.

Police officials say authorities have erected decontamination tents, in the event further testing come back positive.