Devices that gather data from millions of mobile phones are being flown over the US by the government, according to the Wall Street Journal.
The "dirtbox" devices mimic mobile phone tower transmissions, and handsets transmit back their location and unique identity data, the report claims.
While they are used to track specific suspects, all mobile devices in the area will respond to the signal.
The US Justice Department refused to confirm or deny the report.
The Wall Street Journal said it had spoken to "sources familiar with the programme" who said Cessna aircraft fitted with dirtboxes were flying from at least five US airports.
The department said that it operated within federal law.
Off the shelf
A dirtbox mimics the signals transmitted by mobile phone providers which handsets look to latch on to. When they do, they send their individual registration information and location.
While they are intended to be used to track an individual or small group, all phones within the area where they are operating will also be swept up in the surveillance.
They operate in the same way as Stingray, a more commonly known mobile phone surveillance tool, security expert Prof Alan Woodward told the BBC.
"The governments have access to Stingray which is available off the shelf to spy on mobile phones but governments aren't the only ones. For £2,000 you can build your own," he said.
Tools like Stingray and Dirtbox are known as IMSI (International Mobile Subscriber Identity) catchers because they collect the unique identification data sent by each individual device to its network.
"In essence, you spoof networks by pretending to be a cell tower. You turn off encryption, and then you can extract all sorts of info such as calls made, where, when etc etc.
"'I'm not surprised to hear that it is going on," Prof Woodward added.
"It's easily done. Doing it from the air is the obvious place to do it, but the question is under what legislation they are doing it and what are they doing with the data."