Senators and representatives streamed down the back steps from the House and Senate chambers and waited on the Capitol lawns with staffers, guards and media as the building was searched.

Capitol Police spokesman Dan Nichols said the office of Sen. Robert Byrd, D-W.Va., the Senate's president pro tem, received a telephoned bomb threat about 5:30 p.m. Then bomb-sniffing dogs ''hit on an area that was of concern,'' Nichols said. He would not say whether it was package. A second team of dogs was taken to the same room, and the dogs picked up the same hit, prompting the evacuation, Nichols said.

Police allowed people to return to the Capitol about 30 minutes later, after a team of explosive experts checked out the area and found nothing untoward. The Capitol was evacuated Tuesday after hijackers crashed airliners into the Pentagon and the World Trade Center.

On Thursday, senators were in the middle of a vote condemning discrimination against American Muslims and Arab-Americans when they were pulled off the Senate floor. About an hour later, the Senate passed the resolution, 98-0.

''Unfortunately, this likely is going to become commonplace,'' said Sen. Byron Dorgan, D-N.D., after staffers hustled him from the Capitol. ''This is becoming all too routine given the times in the country.''

The House was in the middle of a secret briefing by FBI Director Robert Mueller; the deputy national security adviser, Steve Hadley; and John McLaughlin, the deputy CIA director, when the evacuation order came.

''Are we frightened? No. Are we taking precautions? You bet. We're going to prevail,'' said Rep. Anna Eshoo, D-Calif.