When a $460 charge at a St. Louis Park Cub Foods store and another charge at a nearby Subway restaurant showed up on his statement, Mike Trafton got suspicious and called police.

Another person who had three charges for $200 each and a third whose statement showed two $200-charges at well-known retail outlets in Bloomington and Richfield also contacted police. The common denominator is that all three people had bought gas at a Mobil station in Robbinsdale.

That’s where on Jan. 19 a gas station attendant found a credit card skimmer inside one of the pumps. Police believe that thieves used the device to steal consumers’ information, then made fake credit cards to make fraudulent purchases.

Crime involving card skimmers has been an isolated problem in Minnesota, officials with the Minnesota Department of Commerce said. But cases in Robbinsdale and another this week in which three men were charged in Hennepin County District Court for using the tiny devices to swipe credit card information from customers at a Bloomington gas station prompted the Commerce Department to issue a warning. Robbinsdale police also issued a crime alert.

The Commerce Department, “as the state’s consumer protection agency, is investigating and responding to this emerging threat,” said Commerce Commissioner Mike Rothman. “It’s important for both consumers and gas station operators to be vigilant.”

Credit card skimmers are small electronic devices that can be attached to credit card readers or hidden inside gas pumps to collect and transmit a customer’s credit and debit card data to criminals, who sell or use the information to commit identity theft.

Robbinsdale police say they don’t know how long the skimmer had been on the pump at the Mobil station on Lake Drive and Lakeland Avenue. Police removed the skimmer and began investigating after people posted stories on social media of odd credit card charges from the Mobil and other places, said investigator Anne Faue. “A lot of time when people have fraudulent charges, they report it to the bank and that’s it,” Faue said. “This has the potential to be a ring of people.”

In the Bloomington case, men from Kentucky attached a skimmer to card readers at Bobby and Steve’s Auto World. Surveillance video showed the men appearing to tamper with a credit card machine on Jan. 22. On Monday, the men returned to the gas station in a white van and again were tampering with one of the card readers. An alert employee called police.

The suspects — Rene Cobarrubia-Costa, Miguel Castillo Fornaris and Brian Miguel Diaz — were arrested. They were charged Wednesday with felony possession of burglary tools and possession of devices with the intent to commit unlawful activity.

According to criminal complaints, police executed a search warrant and found five credit card skimming devices, a credit card reader and a GPS device in the van. Investigators also believe the van was equipped with Bluetooth technology that allows credit card information to be transmitted wirelessly from the skimmer to a computer, the complaint said.

A search of a hotel room in one suspect’s name found a laptop, about 20 credit cards in different names and multiple blank credit cards. Fornaris said he obtained the skimmers in Kentucky and that people there were paying him $500 to obtain credit card numbers, according to the complaint.

It was not immediately clear if the cases in Robbinsdale and Bloomington were related. But Faue said the cases should put consumers on notice.

“This is huge in other places in the country, but we’ve not seen a lot of it here,” she said. “People need to be on the look out.”

Faue said people can still use credit and debit cards at gas pumps, but they should look to see if seals on the pumps have been broken, a sign that a skimmer might be attached. She also said drivers should use pumps closest to the store where clerks can keep a better eye on them. And above all, monitor their accounts.

“None of us sits down with our check register and goes over it with our receipts,” Faue said. “We use and use them [cards] and we look at the statement once in a while. We are not paying as close attention.”

That’s how Trafton spotted his unauthorized charges. His wife, who is a “hawk” when it comes to monitoring the family bank accounts, spotted the unusual transactions. He reported it to the bank and police. He got his money back.

“That could have been somebody’s rent payment or food for their kids,” he said.