SAN FRANCISCO (KCBS)— The Transportation Security Administration (TSA) has noticed a new trend at airport checkpoints where passengers are being stopped for carrying knives that fold up into the size of credit cards.

They’re mostly plastic, but when you unfold the knife, it’s a metal blade, sometimes surgical steel blades that are prohibited on airplanes.

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TSA spokesperson Laurie Dankers said San Francisco International Airport topped the list of credit card knives found on passengers. According to her statistics, 20 of the knives were found this year at SFO; 19 were confiscated at Oakland’s airport.

Across the country, the TSA has confiscated more than 500 of these knives.

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Dan Barrons, a San Mateo resident who flies every week on business said it’s “complete stupidity”.

“That’s all I can say it is. With the heightened security I can’t believe anyone would try,” he said.

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When one of these knives is detected on a passenger, all screening comes to a halt making going through airport security a frustrating process.