TAMPA — They are thin enough to fit inside your wallet. They are sharp enough to slice through cardboard — or to draw blood. And they are apparently popular enough that they're popping up at airports nationwide.Behold the credit card knife, a cutting instrument of growing popularity and the latest cause for concern among transportation security screeners.These lightweight, slim, plastic gadgets are made to look and be carried like your typical credit card, but with a pointed, sheer metal edge smoothly concealed within.The knives have been around for years, but a recent uptick in their appearance at airport security checkpoints has drawn the attention of the Transportation Security Administration. The agency noted the knives June 27 in a weekly blog highlighting the various weapons and potentially harmful objects that security personnel have found at airports around the country."There has been a surge in the last few months," said TSA spokeswoman Sari Koshetz. "Our officers are highly trained to detect threats to the aviation system, including knives such as credit card knives, which passengers have been bringing to the checkpoints in their wallets and carry-on bags."The TSA declined to discuss how the easily concealable blades, which are not unlike pocket knives or box cutters, might be detected in a security screen. The knives turn up in standard X-rays of travelers' personal items and carry-on bags, Koshetz said. And quite a few have turned up in recent weeks.That includes at least eight credit card knives that have been confiscated at Tampa International Airport, and one each at the St. Pete-Clearwater and Sarasota Bradenton international airports. Nationwide, a total of 61 credit card knives were found at airports in June, according to the TSA.The knives, which are always seized, have not resulted in any local arrests, but officials warn of the potential for civil penalties if they are brought to security checkpoints. In most cases, though, there appears to be a lack of any ill intent on the part of the knife-toting public. Among the most popular models is the "Cardsharp" folding safety knife, made by the British company Iain Sinclair. The brand retails online for as little as $3. It is touted as less bulky than a pocket knife, sharp as a surgeon's scalpel and a good tool for food preparation and outdoor utilities.It's hard to say why the knives are showing up at airports with increasing frequency. Doug Ritter, chairman of Knife Rights Inc., an advocacy group for knife enthusiasts, suspects the answer is simply that more people are buying them, carrying them and forgetting that they have them in their wallets."Nowadays there are a bunch of companies that make knives sized to fit in a wallet, whereas 10 years ago there were only one or two," he said. "I think it's simply a niche market pocket knife that's easily carried and easily forgotten."Contact Dan Sullivan at [email protected] or (813)226-3386. Follow @TimesDan.