WASHINGTON —  The House of Representatives (search) 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 and lawmakers went back in session.

U.S. Capitol Police Chief Terrance Gainer (search) 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 Cannon House Office Building (search) in pursuit of suspects.

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Gainer said the two female 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.

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

The incident triggered memories of a 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.

Rep. John Shimkus, R-Ill., 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. Shimkus' office would not identify the two aides and no charges were filed.

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

Shimkus said the staff members didn't realize they were the suspects until Capitol police said they were looking for a woman.

"This was an unfortunate misunderstanding, a result of my staff's efforts to put together a Halloween costume during their lunch hour," he said in a written apology.