For the last seven years, the U.S. Marshals Service has been using electronic devices installed in airplanes to track individuals.

Known as “dirtboxes” by law enforcement, the two-foot-square devices are said to mimic cellphone towers and trick handsets into sending over their unique registration information according to people familiar with the matter as reported by The Wall Street Journal.

Sources claim dirtbox-equipped Cessna aircraft operate out of at least five metropolitan-area airports on a regular basis and have a range that can cover most of the US population. A single flight is capable of collecting data on thousands of cellphones. The data can be used to pinpoint a phone’s location to within 10 feet, even down to a specific room in a building.

Law enforcement use the technology specifically to locate phones belonging to individuals under investigation such as drug dealers and those suspected of murder. As you might expect from such a dragnet program, much of what is collected belongs to people that aren’t suspects. Once the device determines which phones belong to targets, they “let go” of non-suspect phones, sources said.

Revised versions of the technology are even more impressive as they can pull off other feats like jamming a signal or even scrape text messages and photos from a device. It’s unclear if these capabilities have been used over US soil, however. Abroad, the technique is used by military and intelligence officials to help locate suspected terrorists.

Sources say what is done over US soil is completely legal although whether or not it should be done is a separate question.