(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.

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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.

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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.

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Protestors in other cities have reported the same thing.