According to a new study, five Nigerian criminal gangs are behind most scams targeting sellers on Craigslist. In a new twist, they are using professional check-writing equipment plus U.S.-based accomplices to not raise suspicions among their victims.

Damon McCoy, an assistant professor in the computer science department at George Mason University, said: “I think the most surprising thing was the number of people in the U.S. participating in this scam.”

McCoy and colleague Jackie Jones, of George Mason’s information technology department, seeded Craigslist with offers to sell laptops. The bogus buyers contacted them via email. McCoy and Jones replied with an email containing an image of the laptop. When the link was clicked on, the scammers real IP address was revealed.

The IP addresses all returned to Nigeria.

If you are familiar with the scam, it works like this: the buyer sends the seller a check for well over the amount for the laptop. Often the check, written by professional check writing equipment, is to the tune of $1500. The seller is supposed to cash the check and send the excess funds to a third party. The same third party that the seller is instructed to send the laptop to.

What was particularly interesting about this scam is that the checks were all sent from within the U.S., indicating that the groups in Nigeria recruited local help.