NBC News reports that online retailers and drone technology companies are marketing RF jammers as drone deterrents or privacy tools, thereby circumventing laws that prohibit the sale of such devices in the United States.

The warning issued by the Federal Communications Commission (FCC) states that signal jamming devices can block emergency calls and pose a serious risk to public safety communications, while also interfering with other forms of everyday communications and air navigation systems.

The FCC said: "It is a violation of federal law to use phone blocker, GPS blockers, or other signal jamming devices designed to intentionally block, disrupt, or interfere with authorized radio communications. Use in businesses, classrooms, residences, or vehicles is not exempt. Local law enforcement agencies do not have independent authority to use jamming devices; federal law enforcement agencies may use them under applicable regulations under certain limited exceptions."

The FCC warning also states that "it is illegal to advertise, sell, distribute, import, or otherwise market jamming devices to consumers in the United States"

But NBC reports that Amazon third-party sellers, independent online stores based in China, and small domestic companies that specialize in drone-related equipment are all ignoring the law. The FCC told NBC that it is investigating the sale of jammers, including on Amazon.

As part of its investigation, NBC spoke with the CEO of a U.S. company that offers portable anti-drone radio frequency jamming devices online. The CEO told NBC that the devices are easily available if consumers have the money, but they mainly see interest from large companies and government agencies as they prepare for domestic drone terrorism.

There are more people selling "jammers." The devices can block cell phone calls, text messages, Wi-Fi networks and GPS systems and can be used to cause chaos in public places.

The small, battery-powered devices can be used to create "blind spots" in a small area (usually around 30 feet) and have been used by movie theaters, restaurants and schools to prevent people from using their phones. But they can also cut off 911 calls, interfere with navigation near airports, and have been used to disrupt radio communications near police stations. FCC officials said they have noticed an increase in the flow of jammers, which are banned by federal law, into the United States. Many of the cheaper versions, which sell for as little as $25, are imported from Asia, according to the agency.

It is illegal to sell, advertise, use or import jammers under the Communications Act of 1934, which prohibits blocking radio communications in public

The FCC said cities including Orlando, Philadelphia, Austin, Mississippi, Charlotte, North Carolina, Washington, D.C., Cincinnati and Corpus Christi, Texas, advertised jammers on the site. Officials said they do not believe the cases are related.

"Simply posting an ad for a signal jammer on a site like Craigslist.org is a violation of federal law. Signal jammers are contraband for a reason," Michele Ellison, director of the FCC's enforcement bureau, said in a statement. "One person's moment of peace or privacy could very well jeopardize the safety and well-being of others."

Most sellers advertised jammers as a way to take an "undisturbed nap" on a bus, quiet a classroom or keep your area "free of interference," without mentioning the more nefarious uses the device could be used for, according to the citations.

"We are increasingly concerned that individual consumers operating jammers do not appear to understand the serious consequences of using jammers," one citation reads. "Instead, these operators mistakenly believe that their illegal operations are personal convenience or should be excused."

Keith Grabowski allegedly advertised on Philadelphia Craigslist for a "cell phone jammer, WiFi signal jammer" for $300. He stated in the ad that "due to the nature of this item, we are disclosing very few details," that the jammer was "not a toy," and that "I just want to get rid of it as quickly as possible."