The FCC’s Enforcement Bureau has taken action against 20 online retailers for selling illegal cell phone signal jammers and other equipment that blocks GPS and Wi-Fi signals.

The companies have been ordered to stop selling the devices in the United States and have been asked to divulge their suppliers, distributors and manufacturers. The combined companies were selling more than 200 different jamming devices.

The retailers have not been fined, but a second violation could lead to financial penalties between $16,000 and $112,500, with additional charges for continuing to sell and market the devices.

The FCC said the crackdown was intended to warn retailers and potential purchasers that they could face considerable consequences for selling and using jammers.

“We will not tolerate continued violations of federal law,” FCC Enforcement Bureau Chief Michele Ellison said in a statement. “Jamming devices pose significant risks to public safety and can have unintended and sometimes dangerous consequences for consumers and first responders.”

Jamming devices indiscriminately interfere with communications signals, making them especially problematic when used in public places. For example, a jammer used to stop students from using cell phones in a classroom would also block nearby wireless calls to 911.