Les Goldsmith, CEO of ESD America, the makers of the secure Cryptophone 500, said it is unclear who installed the units.
"It's highly unlikely that federal law enforcement would be using mobile interceptors near the Senate. My suspicion is that it is a foreign entity," he said.
The devices, which aren't really towers and can be as small as a suitcase, can listen in on phone calls. VentureBeat, which first reported on the devices, said two kinds of interceptors have been found: "IMSI Catchers, which can steal subscriber info for later monitoring, and GSM Interceptors, which can listen into calls." Some devices do both.