Activists say the Chicago Police Department is monitoring their cell phones at protests, and they are trying to use a lawsuit to prove it.

At issue are cell-phone interceptors called stingrays. These force mobile phones to communicate with them by mimicking a cell tower. For years, Chicago police denied owning any of these stingrays, but a 2014 lawsuit forced the department to turn over records proving the department had purchased several of them.

Government transparency attorney Matt Topic was the lead attorney on that case.

“Once the stingray has tricked phones in the vicinity into thinking it’s talking to a cell tower when it’s actually talking to the police,” Topic said. “It can force the phones to broadcast to the police things like … the call logs and many think these can actually be used to intercept the content of the communications themselves.”

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Topic, who is an attorney at Chicago firm Loevy and Loevy, says because the interceptor mimics a cell tower it can only work within a certain radius.

“I don’t know what that radius is, but I believe it’s a large enough radius that if the police department put one of these devices into a truck or into a car and drove it up next to a reasonably sized protest, they could certainly secretly obtain a lot of information from protesters who are there.”

Topic says it is a reasonable concern that the Chicago Police Department may be using these stingrays to get information during demonstrations. He pointed to a post by the hacker group Anonymous of a recording allegedly taken from the Chicago police scanner. In the recording a man, who Anonymous says is a Chicago police officer, asks if the department is monitoring a protest organizer’s cell phone conversation.

That recording, and pictures of an Office of Emergency Management and Communication vehicle allegedly following marchers, has sparked several allegations of police spying.

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An OEMC spokeswoman says the SVU that has raised demonstrators’ suspicious is a vehicle equipped with mobile field cameras the city uses often in planned and unplanned large scale events and that it’s nothing more. She says it does not have any sort of spying or monitoring capabilities beyond the ability to shoot video.

And a Chicago Police Department spokesman says the department hasn’t used the stingrays during demonstrations.

But Ed Yohnka with the American Civil Liberties Union of Illinois says that isn’t good enough. Yohnka says the department refuses to admit how they are using the stingrays, which naturally leads people to be suspicious.

Yohnka says their use has been “treated as a great secret by government at all levels.”

“We know that this technology has been used in connection with protests in other places,” Yohnka said. “I don’t know whether or not that’s been used in Chicago. I would say that if this technology is being used to track people, if there are technologies that are being used to collect large swaths of communication, those are things that are very troubling and very wrong and I think people would rightly be concerned about them.”

Topic’s latest lawsuit, brought on behalf of privacy advocate Freddie Martinez, is meant to compel the Chicago Police Department to say when and where the stingrays are being used.

“The second suit, which asks for broader records (as to the extent to which the equipment is being used, with what constitutional safeguards, what happens with data), that complaint was filed a while back and we’re expecting the police department’s answer to that complaint [this] week,” Topic said.

“We’re very interested to see … whether these are wholesale constitutional violations and if so we intend to explore what can be done about them,” Topic said.

Patrick Smith is WBEZ’s morning news producer. Follow him @pksmid