U.S. authorities have charged 20 people, including more than a dozen Romanian citizens, with running a multimillion dollar scam that targeted Americans by pretending to sell cars online.

The sprawling case announced Thursday by the Justice Department began in a small Secret Service office in Lexington, Ky., and grew to uncover what officials said was an international racketeering and money laundering scheme.

Officials said the case is a warning to consumers to be wary of increasingly nimble online scammers.

“The defendants allegedly orchestrated a highly organized and sophisticated scheme to steal money from unsuspecting victims in America and then launder their funds using cryptocurrency,” said Assistant Attorney General Brian Benczkowski.

According to court documents, the suspects posted fake ads on auction and sales websites like Craigslist and eBay, advertising cars and other high-cost items. Often, the cars were advertised as being for sale by members of the U.S. military who were soon going to deploy overseas. In truth, according to the indictment, the “sellers” were in Romania and had nothing of actual value.

The suspects also tried to make their offers look legitimate by pretending to be affiliated with eBay and other companies, giving assurances that any financial transactions were secure and could be refunded, according to authorities.

The indictment charges that once a victim sent money to purchase a car, that money was quickly traded for bitcoin so the money could be moved out of the United States.

A dozen of the suspects have been extradited to the U.S. to face trial, officials said.