Fake mobile phone towers of unknown origin have been discovered across America and could be listening in on unsuspecting callers.
A report in Popular Science magazine claims that the mysterious towers have the ability to attack mobile phones by eavesdropping and installing spyware.
The towers were discovers by people using CryptoPhone 500, a heavily modified Android device that tells the user if their phone is being subjected to a baseband attack. The software also allows the user to locate the source of the attack.
The fake towers were originally detected in July, but it is thought that there could be more located across the US.
Chief executive of security firm ESD America, Les Goldsmith, told the magazine that the number of interceptors is higher than expected, with many located close to military bases.
"What we find suspicious is that a lot of these interceptors are right on top of US military bases," he said.
"So we begin to wonder - are some of them US government interceptors? Or are some of them Chinese interceptors? Is it just the US military, or are foreign governments doing it? The point is: we don't really know whose they are."
The interceptors are sometimes disguised as fake cell phone towers, but could also be small mobile devices that deceive phones into giving up information. These smaller interceptors are known as "stingrays" and are usually more difficult to detect.