Kimberly Schneider, police spokeswoman, said: “We don’t have a nerve agent.”
She said all those evacuated would be allowed to return to the Russell Office Building after the three-hour security scare.
An initial test at the Russell Office Building was positive, although a second test on the agent was negative, CNN reported on Wednesday.
Capitol Hill police told Reuters about 200 senators and staffers were evacuated to a nearby car park.
Among the seven senators quarantined were Christopher Dodd, a Democrat from Connecticut, and Chuck Hagel, a Republican from Nebraska.
Eileen McMenamin, a spokeswoman for for Senator John McCain, told the CNN: “There was an alarm to immediately leave the building.
“There are guys down here testing the air. They just used the words ‘nerve agent’.”
She said she was speaking from a basement area of the office building.
CNN said a third test was under way and the result was expected about 8.40pm EST (0140 GMT).
The last major scare in the Capitol occurred about four years ago when tests were carried out for suspected anthrax.