The FBI is operating a small air force of surveillance planes flying across the US and registered under fake company names.
The low-flying aircraft are equipped with video cameras and technology that can identify mobile phone users thousands of feet below, even if the user is in their own home rather on the street.
The disclosure, revealed in an Associated Press investigation, raises fresh questions about the scale of domestic surveillance by the US government and its tendency to gather information on Americans who are not suspected of any crime.
It comes as the US Senate debates a bill intended to stop the National Security Agency (NSA) from collecting the phone records of millions of Americans under one of the controversial programmes exposed by Edward Snowden.
A standoff between Republican senators meant that Congress allowed the Patriot Act, the sweeping anti-terror law passed after September 11, to lapse on Sunday night without voting on a bill to replace it.
The White House urged senators to swiftly pass the reform bill, known as the USA Freedom Act, but hawkish Republicans are trying to amend it to strengthen the hand of the NSA and slowing down its path.
The Associated Press found that the FBI was operating at least 13 fake companies to disguise it secret surveillance fleet, and registered them under generic names like FVX Research, KQM Aviation and NBR Aviation.
Surveillance planes were found to have made more than 100 flights in the last month over at least 11 states and major cities like Boston, Seattle and Houston.
The FBI insists the aircraft are "not equipped, designed or used for bulk collection activities or mass surveillance".
The surveillance aircraft, mainly small Cessnas, can be equipped with a technology that acts as a mobile phone tower and essentially tricks mobile phones into giving up information about their location.
Flights over crowded population centres could pick up on thousands of phone records of people who simply have their mobiles next to them in their own homes. (© Daily Telegraph, London)