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BOULDER COUNTY, Colo. — The discovery of skimming devices at two Boulder County gas stations has led to one arrest, but investigators believe there are more victims who have yet to learn their cards have been compromised.

Boulder County Sheriff’s Office investigators arrested 37-year-old Art Appolon at the Boulder Target store on one count of felony theft and 80 counts of identity theft after the store caught him trying to buy gift cards with cloned credit cards linked to skimming devices found at two area Conoco gas stations.

The stations are less than a mile apart. The first devices were discovered at the Conoco station at 191 S.  McCaslin Blvd. in Superior. That led to the discovery of more skimmers at the Conoco station at 520 S. McCaslin Blvd. in Louisville.

Though skimmers are nothing new, the technology, which often includes devices placed on the outside of machines, is getting harder to detect. In this case, the skimmers were hidden away where only employees could find them, the sheriff’s office said.

“Somehow (the thieves) were able to access keys to the gas pumps,” sheriff’s spokeswoman Heidi Prentup said. “They were able to open up the gas pumps and put the skimmers on the inside so victims wouldn’t have even known that it happened.”

There have been 136 cardholders identified so far, but that is likely just a fraction of the people impacted. Investigators don’t know how long the skimmers were in place before they were discovered.

In addition to the arrest of Appolon, there is an arrest warrant for Anthony Armando Hernandez, 25 of Miami, who is also believed to be involved. He is wanted for one count of felony theft and 80 counts of identity theft, but he has likely fled the state, the sheriff’s office said.

“It looks like these people are traveling across the United States doing this and it’s definitely organized,” Prentup said. “There is potentially a lot more victims out there.”

To avoid skimmers at a gas station, authorities recommend paying inside or to use the pumps closest to the door. In most cases, including this one, the skimmers were placed on the pumps farthest away from the door of the station so the thieves could avoid being seen.