The Justice Department is scooping up data from thousands of cellphones through fake communications towers deployed on airplanes, a high-tech hunt for criminal suspects that is snagging large number of innocent Americans, according to people familiar with the operations.

The U.S. Marshals Service program, which became fully functional around 2007, operates Cessna aircraft from at least five metropolitan-area airports, with a flying range covering most of the U.S. population, according to people familiar with the program.

Planes are equipped with devices—some known as “dirtboxes” to law-enforcement officials because of the initials of the Boeing Co. unit that produces them—which mimic cell towers of large telecommunications firms and trick cellphones into reporting their unique registration information.

The technology in the two-foot-square device enables investigators to scoop data from tens of thousands of cellphones in a single flight, collecting their identifying information and general location, these people said.

People with knowledge of the program wouldn’t discuss the frequency or duration of such flights, but said they take place on a regular basis.

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A Justice Department official would neither confirm nor deny the existence of such a program. The official said discussion of such matters would allow criminal suspects or foreign powers to determine U.S. surveillance capabilities. Justice Department agencies comply with federal law, including by seeking court approval, the official said.

The program is the latest example of the extent to which the U.S. is training its surveillance lens inside the U.S. It is similar in approach to the National Security Agency’s program to collect millions of Americans phone records, in that it scoops up large volumes of data in order to find a single person or a handful of people. The U.S. government justified the phone-records collection by arguing it is a minimally invasive way of searching for terrorists.

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