DENVER (CBS / AP) — Civil liberties advocates say tools that allow police to search social media in real time during protests could have a chilling effect on free speech.

Police say services such as Geofeedia, which map, collect and store information from social media posts, can be a useful way to help find crime witnesses and spot brewing problems during large gatherings.

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The American Civil Liberties Union objects to the way some departments have quietly unrolled the technology without community input and little public explanation.

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The group’s Colorado chapter sought more information Thursday on how the Denver Police Department uses the program. Legal director Mark Silverstein says he’s concerned it could be used to collect information on law-abiding demonstrators.

Denver police didn’t immediately respond to requests for comment.

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