The FBI is operating a fleet of low-flying surveillance aircraft, equipped with advanced cell phone-monitoring equipment and cameras, across the United States, according to an Associated Press investigation. The small propeller airplanes belong to more than a dozen fictitious companies. An FBI spokesman said they are being used only for ongoing investigations and that the program is not classified or engaged in mass surveillance.

"The FBI's aviation program is not secret," spokesman Christopher Allen said in a statement. "Specific aircraft and their capabilities are protected for operational security purposes" and "are not equipped, designed or used for bulk collection activities or mass surveillance."

However, some of the planes, claimed the report, are equipped with technology that can mimic cell phone towers and force phones to give up basic subscriber information. The FBI say this use of the equipment is rare. And video footage that captures criminal activity unrelated to ongoing investigations can be handed over to assist prosecutors.

Using publicly available information, the AP connected 50 aircraft back to the FBI, and around 100 flights since last April. However, a federal document from 2010 shows the FBI can command at least 65 additional planes, including 90 Cessna aircraft. Over the past few weeks, the AP tracked more than 100 flights in at least 11 states plus the District of Columbia, most with Cessna 182T Skylane aircraft. These included parts of Houston,  Seattle, Phoenix, Boston, Southern California, Chicago and Minneapolis. 

The program, says the government, is being used as a tool to assist in criminal, terrorism or intelligence probes, but it also highlights the growing conflict between current civil liberties laws and the growing use of new, intrusive technologies in government surveillance.  

The news also comes as congressional lawmakers debate renewing the Patriot Act, which expired on Sunday.

"These are not your grandparents' surveillance aircraft," said Jay Stanley, a senior policy analyst with the American Civil Liberties Union, calling the flights significant "if the federal government is maintaining a fleet of aircraft whose purpose is to circle over American cities, especially with the technology we know can be attached to those aircraft."

The FBI flights assisted local police forces in Baltimore during the recent riots that followed the death of 25-year-old Freddie Gray, the AP reported, although an FBI representative told the AP that the surveillance flights were in accordance with FBI rules. The rules, which are heavily redacted in publicly available documents, specify what equipment the agency can use, as well as the duration of and justification for the surveillance.