SAN DIEGO -- Authorities in San Diego are warning the public to beware of the rise in crimes related to credit card skimming.

"It was unnerving," said ABC10 employee Leila Niehuser.

Niehuser's Visa debit card was seemingly safe in her hand nearly two years ago, until her bank flagged her card.

"It took two times to go through," she said.

While Niehuser was visiting family in San Luis Obispo, someone had cloned her card and was making purchases at restaurants and stores in Poway.

"It just made me kinda paranoid because I had no idea where it happened," she said.

Niehuser's case remains a mystery, but detectives said there is a good chance thieves obtained her debit card number at a gas pump.

"These devices are internal; they're hidden," said Sgt. Mark Varnau of the San Diego County Sheriff's Department, referring to a skimmer.

Varnau said skimmer devices are attached to various parts inside the gas pump. While customers are swiping their card, the skimmer records PIN and account numbers.

In February, detectives traced two reports of skimming to a Lakeside ARCO gas station, but the thief was never found.

Sources told 10News there have been at least 12 similar cases around the county in the past year.

Authorities said skimmers have also been found in ATMs, but tight security at the machines, including cameras, have prompted thieves to look elsewhere.

Gas pumps are also enticing because many are opened by universal keys, according to authorities.

"The keys are available online. They can be stolen, keys can be duplicated," said Varnau.

Authorities said once a skimmer is placed inside, there is no easy way to detect them.

Authorities recommend customers go inside to pay, use cash or go to gas stations where the card reader isn't connected to the pump.

Gas stations open 24 hours are better options because it is harder for thieves to break in and place a skimmer, authorities said.

"Ultimately, it's a trade-off between security and convenience," said Varnau.