The FBI's Aerial Surveillance Squadron

The Associated Press reports:

After The Washington Post revealed flights by two planes circling over Baltimore in early May, the AP began analyzing detailed flight data and aircraft-ownership registrations that shared similar addresses and flight patterns. That review found some FBI missions circled above at least 40,000 residents during a single flight over Anaheim, California, in late May, according to Census data and records provided by the website FlightRadar24.com.

What their investigation revealed was a secret squadron of aircraft operated by the FBI to conduct visual and electronic surveillance. Again, from the article:

The FBI is operating a small air force with scores of low-flying planes across the country carrying video and, at times, cellphone surveillance technology — all hidden behind fictitious companies that are fronts for the government, The Associated Press has learned.

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 AP review found.

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[T]he planes can capture video of unrelated criminal activity on the ground that could be handed over for prosecutions.

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 rare.

However, the article points out:

... the AP found in recent weeks FBI flights orbiting large, enclosed buildings for extended periods where aerial photography would be less effective than electronic signals collection. Those included above Ronald Reagan Washington National Airport and the Mall of America in Bloomington, Minnesota.

The article goes on to describe the shell companies used by the FBI to conceal the true ownership of the aircraft.

The article mentions as a possible means to spot some of the aircraft:

Most flight patterns occurred in counter-clockwise orbits up to several miles wide and roughly one mile above the ground at slow speeds. A 2003 newsletter from the company FLIR Systems Inc., which makes camera technology such as seen on the planes, described flying slowly in left-handed patterns.