U.S. Capitol Police Chief Terence Gainer told reporters the incident resulted from "two staff members bringing in Halloween costumes....I don't think they had any ill intent," he said.
He said the two female staff workers stopped to chat with security personnel after placing a bag on a security station X-ray belt at the entrance to the Cannon House Office Building, then went into their building. Moments later, security officials noticed the image of a gun, and triggered an alarm.
Gainer said the two staff aides were "very sorry this all happened" and that the security personnel had performed "well within standards."
The perceived security breach triggered an immediate massive response.
Police invaded the office building, mounting a room-by-room search for the suspect or suspects in the case.
Two or three officers were quickly stationed at the underground entrances to the Capitol from each of several House office building. Outdoors, security barriers were raised to prevent vehicles from approaching buildings in the Capitol complex. Police in SWAT gear also materialized.
In the chaotic moments after Thursday's incident, the congressman who chairs the committee overseeing House administration procedures told reporters that the image seen by security personnel "could be a toy gun."
"But it could be real," said Rep. Bob Ney, R-Ohio.
Not long after a review of security videotapes, a police spokeswoman, Jessica Gissubel, told reporters, "We do now believe it is a revolver."
Then, another 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.
The names of the two staff aides were not disclosed.
Rep. Philip Crane, R-Ill., chairing a committee meeting at the time the incident occurred, interrupted the hearing to announce that the alert had been lifted.
"It was a Halloween prank. A very stupid thing for whoever did it," he said.