Fourth of July travel? Secret Service warns of gas pump skimmers

Planning a road trip for the Fourth of July? Be extra careful when putting your credit card into the gas pump. That's the warning from the U.S. Secret Service this year.

Nearly 47 million Americans are expected to hit the road to celebrate what promises to be the busiest Fourth of July holiday weekend in history, according to AAA.

That's why Secret Service agents from 36 field offices will be out locating and recovering skimming devices from gas stations in 21 states during the holiday week, the agency said Tuesday.

Skimmers are devices that detect and record credit card information at payment terminals, such as gas pumps. The devices are are getting smaller, smarter and harder to detect. 

Gas stations are a prime target during holiday weekends.

"Because today's gas pumps are typically unattended, developing suspects and making arrests in skimming cases is difficult - but not impossible," the agency said in a statement.

The Secret Service recovered more than 70 skimmers when they conducted a similar campaign over the Memorial Day holdiay this year.

Credit card thieves are getting smarter. Card skimmers have gotten so advanced, even experts may be fooled. But there's a way to spot at least some of them.

Tips to avoid getting scammed at the pump. Here's how to stay safe.