The FCC Enforcement Bureau is cracking down on sales of illegal cell phone signal jammers on Craigslist.

Six people were cited yesterday for advertising and selling the devices on the popular website, part of the commission’s undercover effort to root out illegal signal jammer sales.

“Merely posting a signal jammer ad on sites like craigslist.org violates federal law,” FCC Enforcement Bureau Chief Michele Ellison said in a statement. “Signal jammers are contraband for a reason. Simply put, jammers block all communications within range—not just the unwanted ones.”

Signal jammers are indiscriminate and block all communications, including critical services used by the police and other emergency personnel. It is illegal for consumers to use, import, advertise, sell or ship a device that interferes with authorized communications on private or public property.

The commission stepped up its enforcement efforts after noticing an increase in the sales of the devices on Craigslist and other online marketplaces. Over the past two weeks, the FCC has targeted 23 Craigslist ads for signal jammers.

So far, the FCC has largely held off on fining individuals caught selling jammers. However, it is now warning that it will “impose substantial monetary penalties for similar violations going forward.” Fines go up to $16,000 for each violation, with a maximum fine of $112,500 for individuals who fail to stop selling, advertising or using the jammers.

The FCC has launched a toll-free tip line for the public to report the use or sale of signal jammers, 1-855-55-NOJAM (or 1-855-556-6526). It also released a consumer alert in English, Spanish and Mandarin Chinese.