The Wall Street Journal reported today that the United States Department of Justice has been using planes equipped with devices that pose as cellular towers (called “dirtboxes”) to collect data from suspected criminals’ cell phones—and capturing data from innocent bystanders in the process.

The devices are capable of capturing data from “tens of thousands” of phones over the course of a single flight. Because most cell phones are designed to automatically connect to the tower with the strongest signal, these dirtboxes can easily fool phones into latching onto its signal.

Once a phone has connected to the dirtbox, it will send its unique identifying information to be compared to determine whether it belongs to a suspected criminal. If it does, the plane will then use the signal strength to determine the location of that individual with an accuracy of within three meters, the report says.

Newer versions of the device can go even further by jamming cell signals or pulling texts, photos, and other data from the phone.

If the phone is determined to belong to an innocent bystander, the device is supposed to ignore it, but the actual safeguards put in place to ensure that no data is collected from innocent targets is stored are unknown.

Some at the Department of Justice have questioned whether this type of surveillance is legal, noting that judges who sign warrants allowing for the location of a suspect’s phone may not understand that doing so may also involve collecting data on countless other people.

The Journal notes that even on-device encryption like Apple employs on newer iPhones isn’t enough to defeat the tactics employed by these devices.

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