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'."
CNN said a third test was under way and the result was expected about 8.40pm EST (0140 GMT).
She said she was speaking from a basement area of the office building.
The last major scare in the Capitol occurred about four years ago when tests were carried out for suspected anthrax.