Last month, thieves at Walmart stores in Virginia and Kentucky used skimmers -- devices placed over credit card readers -- to gather information and pin numbers and make duplicate cards.

A skimmer works by recording the digital information from a swiped card, and a micro-camera captures the pin number. Those are sent to the thief via Bluetooth or removed later.

In a security video, one man shields his partner, who pulls the skimmer out of his jacket and pops it in place. The whole operation takes two seconds. They made off with as much as $20,000 from at least 38 victims.

A thief places a skimmer over a credit card reader at a Walmart, while a friend shields him. CBS News

Michael Seremetis is with the Secret Service, the agency which investigates these types of crimes.

"They download the information and put it on a duplicate card. Either a gift card or another credit card they have," Seremetis explained. "They would download the information onto this magnetic strip, and they're able to use it for any type of fraudulent purposes."

The devices are identical to the real things and have plagued gas stations and ATM machines for years. Crooks have now gone retail, using them on self-checkout machines at those Walmarts, and separately at Safeway stores in California and Colorado.

Security video from a convenience store in Miami showed that it took mere seconds for the clerk to be distracted, and a skimmer placed over the credit card scanner.

Bob Sullivan is a security expert with Credit.com.

"It's remarkable how fast these things can be put on top of a cash register. The folks who do the credit card skimming -- they're basically magicians. They use slight of hand and in an instant, they can plop one of these things onto another terminal almost invisibly," Sullivan said.

Walmart now requires customers to use special chip-enabled cards, more secure since they're not swiped. But 30 percent of credit cards don't have them. And only 20 percent of card terminals are compatible.

The Secret Service says you can protect yourself by always covering the key pad with your other hand when you enter the pin, and check your accounts daily for any suspicious charges.