Police and other officials in the U.S. Capitol are vowing to tighten security after a scare forced the shutdown of a congressional office building.

Since the September 11, 2001 terrorist attacks, security on Capitol Hill has been gradually tightened.

New barriers block off the Capitol building itself. The special capitol police force is on high alert around the clock.

But when incidents do happen, they spark instantaneous reviews of security procedure.

Early Thursday afternoon, guards at an entrance to one of the House of Representatives office buildings noticed what appeared to be the image of a gun in a knapsack going through an X-ray machine.

As the story was first told by police, a white male grabbed the sack and ran. An immediate alert was issued, and police ordered a "lockdown" of the Cannon Office Building, not allowing anyone in or out.

Heavily-armed SWAT, or special tactics police, entered the building and for a while nobody, especially hundreds of reporters and television crews gathered outside, knew much of anything.

Until, that is, Capitol Police Chief Terrance Gainer came to the microphones. "The incident that started a couple of hours ago resulted from two staff members bringing in a Halloween costume that included a plastic weapon," he said.

The holiday of Halloween takes place Friday and is an occasion for dressing up in costumes.

The two congressional staff aides who brought in the toy gun were described as "very sorry" at having triggered the security alert.

Police chief Gainer said the security officers under his control handled the situation properly, a view echoed by Congressman Bob Ney, who chairs a committee overseeing capitol security.

"The Capitol Police did a fine, effective job, they were also in the other buildings going room to room," he said. "This could have been a real weapon. Every single day as we know very well in the Capitol Hill family here, they lay their lives on the line."

However, later Congressman Ney issued a statement saying the incident raised some very serious questions, and vowing to examine carefully how it happened.

The incident underscores the sensitivity of anything that even approaches a security breech on Capitol Hill, in these days of continuing nervousness about possible future terrorist attacks.