Feb. 8 (Bloomberg) -- A ``suspicious substance'' found in the U.S. Senate's Russell Office Building in Washington late today was determined to be non-threatening, and workers were allowed to return to their offices, Capitol Police said.

The Russell building, about a block north of the Capitol, was evacuated for roughly three hours after an alarm went off shortly before 7 p.m. indicating the presence of a nerve agent.

``We had an initial alarm in the Russell attic building that detected a nerve agent,'' Sergeant Kimberly Schneider of the Capitol Police said at a news conference. Several tests were run on a substance found in the attic, she said. An initial test suggested a nerve agent, and subsequent tests turned up negative.

More than 200 people, including about a dozen senators, were evacuated and held in a nearby underground garage while the threat of exposure to chemical agents was determined, Schneider said. There were no reports of injuries.

Senator Jeff Sessions, an Alabama Republican, was among those evacuated and spirited to the garage.

``This warning system worked,'' Sessions told reporters after being released. ``People in the building followed the directions properly.''

He said the mood during the evacuation and in the garage was calm, and the police told them they had to stay until authorities made sure there were no problems. ``People were hungry,'' Sessions said, but ``nobody was complaining.''

Heightened Security

Security has been heightened at all government buildings since the Sept. 11 terrorist attacks in 2001 and after anthrax- tainted mail was sent to government officials in Washington and media organizations in New York City and Florida weeks later.

The Capitol and the adjacent office buildings have been evacuated several times because of alerts that turned out to be false alarms, including those caused by small aircraft that strayed into the restricted airspace over Washington.

Senators and their staff have offices in the Russell building and adjacent facilities that are connected to the Capitol chambers by an underground tunnel. House of Representatives members have offices south of the Capitol.

Schneider said the attic area where the suspicious substance was detected is primarily used for storage. She wasn't able to provide a conclusive answer to what the substance was or why the alarms went off.

To contact the reporters on this story: Ken Fireman in Washington at Kfireman1@bloomberg.net; Kristin Jensen in Washington kjensen@bloomberg.net

Last Updated: February 8, 2006 22:03 EST