In a case dating back more than 4 years, the U.S. Federal Communications Commission (FCC) has ordered retailer HobbyKing to pay a nearly $2.9 million fine in connection with the company’s marketing of drone transmitters that operated in unauthorized radio frequency bands.

According to a Forfeiture Order issued by the FCC in late July, the company “advertised and sold on its website to U.S. consumers dozens of models of auto/video transmitters for use with unmanned aircraft systems (drones), without regard to whether those AV transmitters were compliant with the…Commission’s rules.” The devices reportedly provide a video link between transmitters mounted on drones and drone users but can operate outside of frequency bands designated for amateur use, thereby requiring FCC certification.

The FCC’s Spectrum Enforcement Division initially investigated HobbyKing in 2016 after receiving multiple complaints regarding the company’s marketing of the illegal transmitters. Subsequent investigations by the FCC determined that HobbyKing marketed at least 65 different transmitter models that had not been FCC certified.

More troubling, 12 of the models marketed by the company operated in restricted frequency reserved for federal uses and could interfere with critical systems of the Federal Aviation Administration (FAA) and other operations. In addition, three of the models were found to operate at power levels exceeding Commission limits and that could interfere with FAA terminal doppler weather radar.