(CBS) -- Activists who have been protesting in Chicago claim they have proof that police have been using so-called "Stingray" technology to eavesdrop on their phones, reports WBBM's Mike Krauser.
The technology essentially puts up a wall between the user's phone and their provider, forcing phones in the immediate area to send data to the police instead of the nearest cell towers.
Activists have posted photos online of a City of Chicago marked emergency management vehicle with what looks like radar on top following protestors.
In October, the Chicago Police department acknowledged that it had purchased cell-phone interceptor devices back in 2008. This included IMSI Catchers, sold under names like Stingray -- hence the name.
The activists think they've found proof police are using Stingray technology in a radio exchange between officers on the street and headquarters during a recent protest.
Dispatch: "CPIC [Chicago police's spy 'fusion' center] on the air for a mobile"
Officer 1: "Go ahead"
Officer 2: "Yeah one of the girls, an organizer here, she's been on her phone a lot. You guys picking up any information, uh, where they're going, possibly?"
Officer 1: "Yeah we'll keep an eye on it, we'll let you know if we hear anything."
Officer 2: "10-4. They're compliant, and they're, they're doing ok now but she's spending a lot of time on the phone."
Officer 1: "10-4"
Some protestors have said that when the OEMC vehicle, which they believe to be Stingray equipped, was nearby, their phones weren't working properly.
Protestors in other cities have reported the same thing.
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