Police are still trying to determine what triggered an air monitoring alert on Capitol Hill Wednesday night.
The false reading for a nerve agent could have been set off by something as innocuous as cleaning fluid.
But until tests prove negative, about 200 people were evacuated from the Russell Senate Office Building into a parking garage, including nine senators.
Majority Leader Bill Frist, who also happens to be a doctor, says the alert was prompted by a single sensor and that no suspicious chemicals were found.
The last time lawmakers were quarantined on Capitol Hill was two years ago when a deadly poison was found in Frist's office.