Secret Service warns of gas pump card skimmers as July 4 travel hits The agency says card skimmers are hidden inside the pumps.

As Fourth of July travel heats up, the U.S. Secret Service is warning motorists, businesses and police of a disturbing surge in criminals’ stealing credit information at gas pumps.

Fraudsters are installing a tiny device called a skimmer inside the gas pump.

The device copies credit card information when customers pay.

"So what happens is you go to your local gas station, you put in your payment card, you pump your gas and you drive home,” Matthew O'Neill, assistant to the special agent in charge of the Criminal Investigative Division, said. “But in what's in reality happening is during that transmission process, a skimmer is acting in between where you put your card in and the point of sale terminal that's transmitting the data to a financial institution and they're stealing your payment card numbers.”

This past weekend, the Secret Service fanned out across the nation — at more than 80 locations in 21 states — hunting for the devices.

The Secret Service has recovered 59 skimmers from 85 locations during the initiative.

Once criminals have the credit card information, they are able to print fraudulent credit cards by the thousands and use those cards to make purchases.

"They're monetizing the stolen payment card data in multiple ways and the easiest way is they'll take the stolen payment card number and will re-encode a gift card or credit card and then they will use that to buy electronics, gift cards, stuff that they can fence on grey market, the black market or the open market," O'Neill explained to ABC News.

The Secret Service says that millions of dollars every year are being swiped from credit card information obtained at the pump. It says one reason it is so easy is it can be done so quickly.

"It will take just a matter of minutes depending on the level of tradecraft that the fraudster has," O'Neill said.

A high-level skimmer can send credit card information through a Bluetooth device and not have to return to the scene of the crime.

"With a Bluetooth skimmer you don't ever have to retrieve the skimmer if you don't want to because the data is being sent through the Bluetooth device to another internet connected device," O'Neill said.

To reduce the chance of having credit information being stolen, here are some options. Customers can pay with credit cards inside the gas station, rather than at the pump.

And paying with cash may be the most effective way to thwart card theft at the pump.