The Federal Communications Commission has slapped a Chinese electronics firm with a record $35 million fine for selling some 285 signal-jamming devices in the U.S. The company allegedly assured buyers shopping on its website that its products were FCC-approved, when in fact they are banned under federal law in the U.S. and its territories.

Signal jammers transmit radio frequencies that interfere or block communications networks, including Wi-Fi, GPS and other telecommunications. C.T.S. Technology Co. Limited, a Chinese electronics maker and online retailer, has sold the devices to U.S. consumers for two years before FCC agents discovered and shut down its operation.

The fine amount – believed to be the largest in FCC history – is based on the quantity of signal jammers sold. The company was caught when it sold 10 high-powered jammers to FCC agents posing as regular buyers.

“All companies, whether domestic or foreign, are banned from marketing illegal jammers in the U.S.,” according to Travis LeBlanc, FCC’s acting chief of enforcement bureau, American Live Wire reported. “Signal jammers present a direct danger to public safety, potentially blocking the communications of first responders. Operating a jammer is also illegal, and consumers who do so face significant civil and criminal penalties.”

In addition to imposing the fine, the FCC is demanding C.T.S. Technology stop its marketing and sales efforts in the U.S. and turn over information about its buyers in the U.S.

Citizens can voluntarily turn in the jammers and report their use by calling the FCC or filing a complaint online through its website, FCC.gov. Retailers can learn more about the marketing of illegal signal jammers at www.fcc.gov/jammers.