The planes' surveillance equipment is generally used without a judge's approval, and the FBI said the flights are used for specific, ongoing investigations. In a recent 30-day period, the agency flew above more than 30 cities in 11 states across the country, an Associated Press review found.
Aerial surveillance represents a changing frontier for law enforcement, providing what the government maintains is an important tool in criminal, terrorism or intelligence probes. But the programme raises questions about whether there should be updated policies protecting civil liberties as new technologies pose intrusive opportunities for government spying.
"The FBI's aviation programme is not secret," spokesman Christopher Allen said in a statement. "Specific aircraft and their capabilities are protected for operational security purposes." Allen added that the FBI's planes "are not equipped, designed or used for bulk collection activities or mass surveillance."
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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 into coughing up basic subscriber information, is rare.
Details confirmed by the FBI track closely with published reports since at least 2003 that a government surveillance program might be behind suspicious-looking planes slowly circling neighborhoods. The AP traced at least 50 aircraft back to the FBI, and identified more than 100 flights since late April orbiting both major cities and rural areas.