Every breath you take ...

Scores of low-flying planes circling American cities are part of a civilian air force operated by the FBI and obscured behind fictitious companies, The Associated Press has learned.

The AP traced at least 50 aircraft back to the FBI, and identified more than 100 flights in 11 states over a 30-day period since late April, orbiting both major cities and rural areas. At least 115 planes, including 90 Cessna aircraft, were mentioned in a federal budget document from 2009.

For decades, the planes have provided support to FBI surveillance operations on the ground. But now the aircraft are equipped with high-tech cameras, and in rare circumstances, technology capable of tracking thousands of cellphones, raising questions about how these surveillance flights affect Americans' privacy.

"It's important that federal law enforcement personnel have the tools they need to find and catch criminals," said Charles Grassley, chairman of the Senate Judiciary Committee. "But whenever an operation may also monitor the activities of Americans who are not the intended target, we must make darn sure that safeguards are in place to protect the civil liberties of innocent Americans."

The FBI says the planes are not equipped or used for bulk collection activities or mass surveillance. The surveillance equipment is used for ongoing investigations, the FBI says, generally without a judge's approval.

The FBI confirmed for the first time the wide-scale use of the aircraft, which the AP traced to at least 13 fake companies, such as FVX Research, KQM Aviation, NBR Aviation and PXW Services.

"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."

The front companies are used to protect the safety of the pilots, the agency said. That setup also shields the identity of the aircraft so that suspects on the ground don't know they're being followed.

The FBI is not the only federal law enforcement agency to take such measures.

The Drug Enforcement Administration has its own planes, also registered to fake companies, according to a 2011 Justice Department inspector general report. At the time, the DEA had 92 aircraft in its fleet. And since 2007, the U.S. Marshals Service has operated an aerial surveillance program with its own fleet equipped with technology that can capture data from thousands of cellphones, the Wall Street Journal reported last year.

In the FBI's case, one of its fake companies shares a post office box with the Justice Department, creating a link between the companies and the FBI through publicly available Federal Aviation Administration records.

Basic aspects of the FBI's program are withheld from the public in censored versions of official reports from the Justice Department's inspector general, and the FBI also has been careful not to reveal its surveillance flights in court documents. The agency will not say how many planes are currently in its fleet.

The planes are equipped with technology that can capture video of unrelated criminal activity on the ground that could be handed over to prosecutions. One of the planes, photographed in flight last week by the AP in northern Virginia, bristled with unusual antennas under its fuselage and a camera on its left side.

Some of the aircraft can also be equipped with technology that can identify thousands of people below through the cellphones they carry, even if they're not making a call or in public. Officials said that practice, which mimics cell towers and gets phones to reveal basic subscriber information, is used in only limited situations.