Denison Police are investigating after multiple card skimmers were found at a local gas station last week.

In an emailed incident report, Denison Police said officers responded on Sept. 23 to Roger’s Country Store in the 700 block of W. FM 120 after staff discovered the devices inside the pumps. The business’s security cameras showed the skimmers were installed on the evening of Sept. 10.

Card skimmers are often installed in gas pumps or ATMs and extract card numbers and PIN numbers from credit and debit cards.

Card-skimming isn’t the most common crime, but one which can still lead to considerable financial loss and liability.

Here are five things to know about card skimmers from a recent card skimming information event hosted by Douglas Distributing and helpful strategies from Denison Police Lt. Mike Eppler to steer clear of them:

1. Where are skimmers found?

Card-skimmers are typically used at high-traffic businesses and locations that accept credit and debit cards. They’re most often found at gas pumps, ATMs and restaurants.

2. How do they work?

Skimmers record and save a variety of data including card numbers, PIN numbers, zip codes and cardholder information.

The devices can be attached externally to real card slots and are frequently made to match the look of the machine on which they’re used. But when criminals want to keep skimmers out of sight, they may also install them inside of pump card-reading machines.

After leaving the skimmers for some time, criminals return to remove them or download the information recorded. This can be done manually or even wirelessly with a smart phone and Bluetooth connection.

Portable or hand-held skimmers are less common, but use the same wireless technology.

3. What happens to victims’ information?

Card numbers and other information stolen by skimmers are typically used to make unauthorized purchases, especially online.

But criminals can profit even further by selling stolen card numbers to black-market buyers around the world, making the activity more difficult to trace and track. Cardholder information may also be used in part to steal an individual’s identity.

4. Warning signs

Police often recommend that card users visually check any machine they intend to use. External skimmers can be detected and dislodged by simply wiggling the housing around the card slot. Many gas stations will also use security tape or seals on their pumps to show evidence of internal tampering.

If a seal appears to be cut or damaged, do not use the machine.

Suspicious charges, transfers and withdrawals may also point to card-skimming activity, so cardholders are advised to regularly monitor their accounts and transaction histories.

5. Tips for victims

Those who believe they’ve been a victim of card-skimming or find devices are advised to let the the affected business know and contact their local police department. Potential victims should also notify their bank and card companies, and regularly monitor their accounts and transaction histories. A replacement should be sought for any compromised card.