LOUISVILLE (AP) — Federal authorities pointed Friday to multiple arrests and convictions in Kentucky as just the start of a crackdown on credit card skimmers who target gas pumps to steal personal information.

Six cases in the Louisville area alone snared thousands of victims for combined losses exceeding $3.5 million, said U.S. Attorney Russell Coleman. He said the investigation that began in 2015 because of consumer complaints has led to eight arrests and uncovered up to 50 illicit skimming devices at gas pumps in Kentucky’s largest city and some of its suburbs.

Two defendants were sentenced in federal court to multiyear prison terms, while others have pleaded guilty and are awaiting sentencing or will go to trial, authorities said. All the defendants, based out of Louisville, were charged with aggravated identity theft. Other charges included possession of device making equipment, conspiracy to commit mail and bank fraud, money laundering and wire and bank fraud.

Coleman anticipated more arrests as authorities target the growing criminal enterprise.

Joined by representatives of the FBI, the U.S. Secret Service and Louisville Metro Police at a news conference, he warned the penalties would be severe for those caught by law enforcement.

“If you’re going to commit this type of fraud … stay out of the Western District of Kentucky,” Coleman said, “because you will become intimately familiar with the federal Bureau of Prisons.”

The skimming devices, once installed at gas pumps, allow thieves to secretly record customers’ credit or debit card information and steal their identity. Thieves then sell that information on the black market or use it to make purchases or to create fake credit cards, said Amy Hess, the FBI’s top agent in Kentucky.

Perpetrators can use open vehicle doors or accomplices to block clerks or security cameras from noticing as they install or retrieve skimmers, Hess said.

Consumers can take steps to reduce the risk, authorities said. The best protection is to pay for fuel with cash inside the store. Other suggestions include looking for signs that gas pumps were tampered with and using pumps in view of surveillance cameras or store clerks.

“Who has time to analytically examine a gas tank?” Coleman said. “What I’m saying is, we have to. In this day and age and in this climate, take a couple seconds.”