Airport security needs to do a better job of detecting unassembled bombs, the Transportation Security Administration says after an investigation showed bomb-making material had been smuggled through 21 airports.

Kip Hawley, the director of Transportation Security Administration (TSA) said on Friday the agency had already shifted its emphasis from screening passengers and bags for items such as scissors to focus on explosives and potential bombers.

He said results of a secret investigation by the Government Accountability Office (GAO), the watchdog arm of Congress, reaffirmed that priority.

Hawley said the GAO study, in which investigators carried readily available materials that can be used to make bombs through security screening at 21 airports, was limited and on a smaller scale than tests that TSA regularly conducts.

Fears

According to NBC News, which first reported the GAO findings, investigators managed to sneak bomb-making material in at every airport they tried between October and January. It gave few details and said the GAO report was classified.

"In all 21 airports tested, no machine, no swab, no screener anywhere stopped the bomb materials from getting through. Even when investigators deliberately triggered extra screening of bags, no one stopped these materials," NBC said.

The GAO report sparked criticism from lawmakers.

"The fact that government investigators were able to pass through TSA's screening at 21 major airports with bomb-making materials is frightening," said Bennie Thompson of Mississippi, the senior Democrat on the Committee on Homeland Security.

Lawmakers with oversight of aviation matters have long said that most airport security systems cannot detect sophisticated explosives, and have urged the TSA to embrace new technology.

Hawley said the TSA is incorporating new technology, but also needs to improve screener training.

So far, just under half of the 43,000 screeners have been trained to detect bombs and bomb-making materials. The training on the first 18,000 was done by late November - about six weeks after the GAO study began in October.

Hawley said the TSA's mission was not to prevent "any science project" from getting onto a plane.

"It's the terrorist with a real IED that knows what they're doing that can bring a plane down with a high degree of reliability - that's the guy we're looking for," he said.