The FCC says illegal devices that block cell phone signals could pose a security risk.

The FCC has noticed a rise in people selling "jammers." The devices can block cell phone calls, text messages, Wi-Fi networks and GPS systems and could 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 about 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 jam radio communications near police stations. FCC officials say they have noticed an increase in the flow of jammers, which are banned by federal law, into the U.S. 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.

Earlier this week, the FCC issued subpoenas to eight individuals and companies that posted ads for jammers on Craigslist.

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 jammers 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 jeopardize the safety and well-being of others."

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

"We are increasingly concerned that individual consumers operating jammer devices do not appear to understand the serious consequences of using jammers," one of the citations read. "Instead, these operators mistakenly believed that their illegal operations were a matter of personal convenience or should be excused."

But the FCC said at least one seller appeared to know that jammers were contraband.

Keith Grabowski allegedly advertised on Philadelphia Craigslist for sale a "cell phone blocker, wifi blocker" for $300. In his ad, he said that "due to the nature of this item, we are revealing very few details," that the jammer was "not a toy," and that "I just wanted to get rid of it as quickly as possible."

"The nature of the ad suggests that Mr. Grabowski knew about the sensitive and/or illegal nature of the equipment he was selling on Craigslist," the citation reads.

People who receive the ticket have 15 days to remove the ad from the site and provide the FCC with information about where the jammer was purchased and to whom it was sold. Simply posting an ad for the sale of a jammer could result in a fine of more than $100,000.

The FCC has set up a "jammer tip line" for people to report to the agency people who may be selling or using jammers.

"We intend to take increasingly tough enforcement action against offenders," Ellison said. "If we find you selling or operating a jammer, you will be punished."