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Fontana police are warning the public to be vigilant about credit-card fraud following the discovery of a skimming device at a gas station on Monday.

Officers responded to a call reporting the skimmer at a Circle K near the intersection of East Foothill Boulevard and Sierra Avenue (map), according to a news release.

The gas station’s manager had called after noticing the device on pump No. 12, which is located as far from the cashier as possible, police said.

“The suspect used a master key to open the gas pump and install the skimmer,” the release said. “The particular device did not appear to transmit information, which means the information was stored on the device for later retrieval by the suspect. There is no way to tell how long the device had been in place.”

The Circle K location is adding additional security features to each pump to prevent future installation of such skimmers, according to officials.

Skimming devices read the magnetic strip on cards, often with a hidden camera that detects keypad clicks for PIN numbers, the Police Department said. The stolen information can be stored or transmitted to thieves, who then use it to make fraudulent purchases, or to make counterfeit cards for the purpose of withdrawing cash from ATMs.

Authorities encouraged anyone who believes their card information has been compromised to call their creditor or banking institution immediately.