The evacuation at the Russell Senate Office Building on Wednesday night marked the biggest nerve-agent scare yet at the Capitol complex, with officials initially fearing that a deadly chemical substance might have been released in the building, authorities said yesterday.

Terrance W. Gainer, chief of the U.S. Capitol Police, said the initial reading from a sensor in the building "bore a striking resemblance to a nerve agent." Further tests, however, were negative. The alarm prompted the evacuation of about 200 people, including at least eight senators, to an underground garage, where they were held for about three hours.

U.S. Senate staffers are quarantined inside a parking garage near the Russell Senate Office Building Wednesday evening. (By Gerald Herbert -- Associated Press)

Exposure to an amount of nerve agent as small as a drop can cause death within minutes or hours. Gainer declined to identify the agent that appeared to be indicated by the sensor. But the FBI and other agencies that assisted in the emergency response said they were told it was the VX nerve agent.

Officials still were trying to determine yesterday what prompted the alarm. One possibility, Gainer said, was treated lumber that had been brought into the building recently. It could have contained harmless amounts of chemicals that activated the sensors.

"The experts will be doing a lot more analysis of what the heck it is," Gainer said.

Legislators who were evacuated to the nearby Senate parking garage applauded Capitol Police officers for their handling of the alarm, saying they were calm and provided frequent information.

"The police were very, very professional," said Sen. Judd Gregg (R-N.H.). "And you've got to admire them. They go into these buildings, and they don't know what's in there."

Gainer said his force was "100 percent more prepared" than it had been for the crisis brought on by the anthrax mailings to the Capitol in 2001, which killed two postal employees in Washington. Still, he said, his officers were studying Wednesday night's incident to see how they could improve. Among the glitches: Five people, including journalists and staff members, were discovered in the building well after police thought it was cleared, Gainer said.

"I'm trying to ascertain whether we failed to get the message to them, or they failed to heed our message," he said.

The five included two members of an ABC News camera crew, said Emily Lenzner, a spokeswoman for the organization. She said they had been storing equipment in the attic.

"These guys didn't hear a thing," she said.

Sensors placed throughout the Capitol complex occasionally have produced false alarms. Experts working with Capitol Police check the chemical "signatures" picked up by the sensors and often find that they match those of such familiar products as cleaning sprays or paint thinner.

What caused the concerns Wednesday night, Gainer said, was that the chemical signature looked very much like that of a nerve agent. "This was a unique one to us," he said.

Shortly after 6:30 p.m., alarms mounted in Senate offices started beeping loudly, as Capitol Police officers announced the evacuation. E-mails issuing the order were sent to staffers' hand-held communicators.

Senators and staffers described the evacuation as orderly.

"There was no running for the door or anything like that," said Coy Knobel, press secretary to Sen. Mike Enzi (R-Wyo.), both of whom were in the building.

Knobel said the alarms in the hallway outside his office did not appear to be working. However, Gainer said he knew of only one hallway where the public-address message was garbled. "Even if they didn't pick up the electronics, our officers went through the old-fashioned way" to ensure that everyone left the building, he said.

Capitol Police officers decided to take the building's occupants to the Senate garage across Delaware Avenue NE in order to isolate them, Gainer said. But it was quickly obvious that no one in the crowd was showing symptoms of significant nerve-agent exposure, which include vomiting, involuntary defecation, paralysis of the respiratory muscles, unconsciousness and seizures.

Still, Capitol Police officers called in assistance from the D.C. fire department, which set up red, yellow and blue decontamination tents outside the Russell building in case they were needed.