The Chicago Police Department recently started working with a controversial facial recognition tool called Clearview AI.

The department entered into a two-year, $49,875 contract on Jan. 1 with the tech firm CDW Government, according to a statement issued by the department.

The tool collects billions of photos of people from websites like Facebook. The police department says it’s not using the tool for real-time surveillance, only for matching photos for investigations.

However, some advocacy groups still have concerns.

“Before, the Chicago Police Department was just using mugshots from a database to identify people,” said Rachel Murphy, staff attorney at the ACLU of Illinois. “With Clearview, it’s anyone’s social media post — even someone who’s not suspected of any wrongdoing.”

In a statement, CPD said it’s: “using a facial matching tool to sort through its mugshot database and public source information in the course of an investigation triggered by an incident or crime.”

Kristian Hammond, a computer science professor at Northwestern University, said the tool is just providing evidence to the department. He compared it to a witness identifying a bank robber and how the first person identified wouldn’t be arrested.

But one concern is misidentification. Hammond said it’s inevitable and that needs to be kept in mind when creating policy on using this technology.

“That’s the important thing about using technology like this,” Hammond said. “It’s not a for-sure match, it’s a potential person.”