Police in Washington say a major security alert on Capitol Hill turned out to be a false alarm triggered by a Hallowe'en costume.
The alert prompted a huge search of the building
Police initially said they were hunting two intruders armed with a revolver who had breached a checkpoint at an office building belonging to the US House of Representatives.
A security guard at the Cannon Office Building reported that a man had gone through the X-ray machine with a suspicious object in his backpack.
But police later explained that two women staff members bringing in Hallowe'en costumes had caused the panic.
"It was just an unusual set of Halloween circumstances," said
Capitol police chief Terrance Gainer. "I don't think they had any ill intent."
Illinois Republican Representative John Shimkus later apologised in writing for the incident, saying that the two were his aides.
"This was an unfortunate misunderstanding, a result of my
staff's efforts to put together a Hallowe'en costume during
their lunch hour," he wrote.
The BBC's Katty Kay in Washington says that despite the fortunate outcome, questions will be asked about how the two women managed to get even a toy gun into the building without being stopped.
A distracted official had allowed the two women and a man through the security barrier, then noticed an image on the screen of his X-ray scanner that looked like a weapon.
The official reported that a man had breached security, but closed-circuit TV images identified the two women.
After the alert went out and the building was sealed off, the two women realised what had happened and handed over a plastic gun that was part of the Hallowe'en costume.
Police were unabashed by their apparent over-reaction, saying that it demonstrated their ability to discern quickly whether security was at risk.
The Cannon building is one of a number of office buildings that flank the central rotunda containing both houses of Congress.
They are just 200 yards (metres) from the main Congress building. Access to the rotunda is subject to strict security checks.