A toy gun prompted a lockdown on Capitol Hill and sent SWAT teams scrambling to head off disaster.

At approximately 1 p.m. Eastern, Capitol Police launched a room-to-room search of the Cannon Office Building following a report of an intruder, armed with a revolver, bolting the security checkpoint at the southeast entrance to the building.

The revolver was reportedly identified inside a backpack by an X-ray machine.

Cannon Building, oldest part of U.S. Capitol Hill complex

“Someone with a gun made an unauthorized entrance into the Cannon Office Building,” Jessica Gissubel, spokeswoman for the U.S. Capitol Police, said at the time.

The House of Representatives was ordered shut down, with nobody allowed to enter and only those who submitted to an administrative search allowed to exit.

Authorities put out descriptions of two suspects, as specialty teams scoured the hundreds of rooms in the building.

But little more than an hour later, police found their culprits on the fifth floor – two female staffers for Rep. John Shimkus, R-Ill., came forward to clear up the misunderstanding.

They had a plastic revolver in their possession that was intended to be part of a Halloween costume.

”I deeply regret that two of my staff were involved in this incident,” Shimkus, said in a statement late Thursday. ”The staffers wish to convey their deepest regrets to all members, fellow staff, and visitors to Washington who were inconvenienced by this incident.”

The staffers are not expected to be charged.

The scare, which has been described as “a good dry run,” revealed a need for better handling of such a crisis. Congressmen reported receiving conflicting and confusing instructions as to whether to evacuate the building. Others said they were unaware of the situation until outside callers, who saw television coverage, alerted them.

In addition to the X-ray machine being unable to detect that the gun was fake, the initial description of the armed intruder put out by police described him as being a white male with brown hair, weighing 140 pounds, standing 5-foot, 3 inches tall and wearing a black suit and white shirt and carrying a blue-and-gray backpack.

A follow-up bulletin described a second female suspect with long, brown hair, aged 20 to 25 and carrying a black purse and maroon bag.

U.S. Capitol Police Chief Terrance Gainer explained the conflicting reports, saying the officer at the checkpoint was engaged in conversation with the male about his belongings on the X-ray machine when someone else came up and distracted him by asking a question. The two women went through the checkpoint while the officer answered the question.

“In those few seconds, the items passed off the X-ray belt, the parties picked them up and left. And the officer immediately noticed what was very plain on the X-ray machine, something that looked like a weapon and that began the process of trying to discern who that was,” said Gainer.

He explained it was closer inspection of the video surveillance that prompted the second bulletin on the female suspect.

Gainer told reporters he thought the incident “demonstrates our ability to quickly discern that someone could be bringing something in and to resolve it successfully.”

He also said the incident serves as a good lesson for Halloween revelers: “You have to be careful at this time with the costumes you have because obviously we’re all very sensitive about that and it’s not inconceivable that people could be wearing or carrying something that is going to upset folks.”

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