US Capitol police have given the all-clear after a Senate building was evacuated amid fears that a hazardous substance had been detected.
Police ordered staff out of the Russell Office Building
Police spokeswoman Kimberly Schneider said tests on a suspected nerve agent had proved negative.
About 200 people were cleared from the Russell Building and moved to an underground car park.
The evacuation was triggered when an alarm system appeared to detect a poison in the air.
An air-monitoring system indicated the presence of a nerve agent in the attic of the building.
"We had this warning system work," said Republican Senator Jeff Sessions.
"People in the building followed the directions promptly. There was no panic, no running, no upset or anything like that."
A team of specialists investigated the alarm and carried out a series of tests.
Initial readings indicated a nerve agent, but subsequently proved negative.
"Test results have been cleared and all test results are negative, so that's very good news," said Sgt Schneider.
The senators and staff were allowed back into the building about three hours after the scare began around 1830 (2330 GMT).