More than a dozen “fake cell phone towers” could be secretly hijacking Americans’ mobile devices in order to listen in on phone calls or snoop on text messages, a security-focused cell phone company claims. It is not clear who controls the devices.

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ESD America, which markets heavily-encrypted cell phones built within the body of a Samsung Galaxy S3, said it was able to locate numerous towers intercepting mobile communications – but does not know who is running them.

Speaking to Popular Science, ESD America CEO Les Goldsmith recently said that the company has used its phone – the CryptoPhone 500 – to map 17 different fake cell phone towers, dubbed “interceptors,” across the United States. Locations include New York, Chicago, Los Angeles, Seattle, and more.

“Interceptor use in the US is much higher than people had anticipated,” he told the website. “One of our customers took a road trip from Florida to North Carolina and he found 8 different interceptors on that trip. We even found one at South Point Casino in Las Vegas.”

Although these interceptors act as fake cell phone towers, they are not necessary large, physical structures. They could simply be small mobile devices that act exactly like a real tower, deceiving phones into giving up information. Such devices are known as “stingrays,” after the brand name of one popular type of interceptor.

Once connected to a person's phone, they can bypass the mobile device’s encryption to either listen in on calls or capture texts. In some cases, they are also powerful enough to take over a device or make it seem as if it has shut down – only to leave the microphone on in order to eavesdrop.

The American Civil Liberties Union has been attempting to pry information about government and police use of stingray equipment, but its efforts have been met with resistance from federal officials.

According to Ars Technica, the Federal Communications Commission pledged in August to investigate the “illicit and unauthorized use” of interceptors – with a primary focus on foreign governments, criminals, and terrorists – but denied a Freedom of Information Act request to reveal more data about current stingray use.

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For Goldsmith, determining who is using the interceptors is important. He speculated that the operators could be American agencies or foreign governments.

“What we find suspicious is that a lot of these interceptors are right on top of US military bases. So we begin to wonder – are some of them US government interceptors? Or are some of them Chinese interceptors?” he told PopSci. “Whose interceptor is it? Who are they, that’s listening to calls around military bases? Is it just the US military, or are they foreign governments doing it? The point is: we don't really know whose they are.”

Outside of national governments, local law enforcement agencies are also beefing up their stingray/interceptor capabilities. In Oakland, California, police are looking to spend hundreds of thousands of dollars to upgrade their cell phone surveillance system.

Meanwhile, police in Tacoma, Washington have caught the eye of civil liberties advocates after the News Tribune revealed they have been using interceptor equipment to catch cell phone calls for the last six years. Some, including Mayor Marilyn Strickland, said it was legitimate for police to do so as long as people’s rights were not violated. However, the ACLU disagreed, arguing it was like “kicking down the doors of 50 homes and searching 50 homes because they don’t know where the bad guy is.”