WASHINGTON (AP) -- The House of Representatives was ordered shut down Thursday following a reported security breach at a nearby congressional office building, but police later determined that a plastic revolver and Halloween costume were to blame.

U.S. Capitol Police Chief Terence Gainer said "two staff members bringing in Halloween costumes" were responsible. "I don't think they had any ill intent," he said, adding he expected no charges to be filed.

Start to finish, the incident lasted two hours or less - but it triggered a massive security response in a time of terrorism worries. Police in battle gear moved into the House's Cannon office building in pursuit of suspects. Across the street, the House was placed into an unscheduled recess for the first time since the aftermath of Sept. 11, 2001.

Gainer said the staff aides had stopped to chat with security personnel after placing a bag on a security station X-ray belt at the entrance to the office building, then went into their building. Moments later, security officials noticed the image of a gun on a video screen, and triggered an alarm.

Gainer said the two staff aides were "very sorry this all happened," and the security personnel had performed "well within standards."

The police chief was still fielding questions from reporters when the bells rang in the House side of the Capitol, indicating that lawmakers were being called back into session.

The preceding two hours were anything but routine, though. Police hurried into the office building, mounting a room-by-room search.

Two or three officers were quickly stationed at the underground entrances to the Capitol from each of several office buildings. Outdoors, security barriers were raised to prevent vehicles from approaching buildings in the Capitol complex. Police in SWAT gear also materialized.

The incident triggered memories of a far more lethal incident on July 24, 1998, when a man with a history of mental illness rushed into the Capitol and killed two security guards. The accused shooter, Russell E. Weston Jr., is awaiting trial.

In the chaotic moments after Thursday's incident, the congressman who chairs the committee overseeing House administration procedures told reporters the image seen by security personnel "could be a toy gun." "But it could be real," said Rep. Bob Ney, R-Ohio.

Then, a twist.

One lawmaker, whom Gainer did not name, had contacted Capitol police to report that his aides were the two people authorities were looking for. Security personnel raced to the office and learned about the costume and toy gun.

"I don't think anybody was trying to trick anybody. I think it was just an unusual set of Halloween circumstances that unfolded on us," Gainer said.

Even before he pronounced the end of the incident, a handful of lawmakers complained they had learned of it from television, rather than from a security communications network established over the past two years.

"The system didn't alert us at all to any problem," said Rep. Eliot Engel, D-N.Y.

Copyright 2003 Associated Press. All rights reserved.