A false alarm from a security sensor indicating a nerve agent in a Capitol Hill office building prompted officials to quarantine about 200 people, including at least eight senators, in a parking garage.

The all-clear came shortly after 9:45 p.m. EST Wednesday, three hours after an air-monitoring sensor indicated a suspicious substance in the attic of the Russell Senate Office Building.

"Everybody is safe," said Senate Majority Leader Bill Frist. "This was a false alarm." He said the alert was prompted by a single sensor and that no suspicious chemicals were found.

"I'm sure ... there will be a lot of questions about whether we had to be quarantined, and the answer to that is yes," said Frist, who is a physician.

"One of the alarm systems that tests air quality went off with a positive reading, and then it went off again with a positive reading, so I guess they thought it was serious enough that they had to take very aggressive action," said Sen. Judd Gregg, R-N.H., one the senators evacuated from the building.

A spokesman for Sen. John Thune, R-S.D., one of those evacuated to the garage, said people initially tried to leave the area but that police directed them into the underground parking garage across the street.

"We had this warning system work," said Sen. Jeff Sessions, R-Ala., one of those in the garage. "People in the building followed the directions promptly. There was no panic, no running, no upset or anything like that."

Police did not immediately know what triggered the alarm but said it could have been something as innocuous as a cleaning substance.

In February 2004, the deadly poison ricin was found in Frist's office, and while dozens of Capitol employees were quarantined briefly and decontaminated, none of them got sick.

In October 2001, a month after the terrorist attacks, an anthrax-laced letter shut down Congress briefly and closed the Hart Senate Office Building for months of cleaning.

A spokesman for Sen. Larry Craig, R-Idaho, said the senator was not among the lawmakers who were held in the garage, despite initial reports that he was.