BALTIMORE, Md. (WJZ)– A surprising new report from the ACLU details how police monitored social media to arrest people during the Baltimore riots. It includes claims they used facial recognition to find people with outstanding warrants.

Privacy advocates worry surveillance of social media could have a damaging effect on free speech.

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The company Geofeedia claims police used their service to monitor people’s social media accounts in key locations. Even going so far as using facial recognition software to arrest people with outstanding warrants.

“If that’s true, that’s appalling and terrifying,” said David Rocah with the ACLU.

The ACLU uncovered promotional material from Geofeedia that uses Baltimore as a case study. Citing a County officer who was assisting City police.

In the study, the officer claims he used social media track a group of students who “had already hijacked a metro bus and found their backpacks full of rocks, bottles, and fence posts. They planned to do a lot of damage.”

Rocah says he’s concerned about social media surveillance.

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“It is simply not okay for law enforcement agencies to be subjecting us to surveillance without out knowledge and consent,” said Rocah.

City police are already under fire for failing to disclose a new aerial surveillance pilot program.

The Baltimore Sun learned Geofeedia has contracts with police in Baltimore City and County, Anne Arundel and Howard Counties, as well as the city of Laurel.

“Without any public knowledge, input, consent, debate or anything,” said Rocah.

Now the ACLU wants answers about how it’s being used. Baltimore police officials tell WJZ they are not commenting because they haven’t had a chance to review the ACLU documents.

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Facebook, Twitter and Instagram have now revoked Geofeedia’s access to their data.