How Chicago Police Convinced Courts to Let Them Track Cellphones Without a Warrant

The Chicago Police Department has acquired and used several varieties of advanced cellphone trackers since at least 2005 to target suspects in robberies, murders, kidnappings, and drug investigations. In most instances, officers only lightly described the devices’ advanced technical surveillance capabilities to courts, which allowed the police to use them, often without a warrant.

Now, after a lengthy legal battle waged by Freddy Martinez, a Chicago software technician, court orders and case notes were released, painting a more detailed picture of how the second-largest police precinct in the U.S. uses surveillance technology to track cellphones.

Martinez, who leads the Lucy Parson Labs, a Chicago-based nonprofit that advocates digital rights and transparency, originally sued for records in September 2014. He provided the released documents to The Intercept.

The Chicago Police Department did not respond to request for comment.

Cell-site simulators, or IMSI catchers, are typically suitcase-sized devices that emit signals over the wireless spectrum, masquerading as legitimate cellphone towers. When a nearby phone attempts to connect to a tower either to make a call or to check for service, it will instead link-up to the rogue device, beaming back information about its location, its owner, and sometimes the contents of communications.

While tracking one cellphone, the device also often sweeps up information about all the other phones in the area and can disrupt cell service. It’s unclear exactly how long the disruption lasts, or how far the range of each device extends. Activists are currently challenging the Federal Communications Commission to step in and regulate how law enforcement uses IMSI catchers, because they interrupt wireless service without authorization — posing a danger to emergency communications.

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