One evening in April, a California law enforcement officer was browsing Facebook when she saw a post from the National Center for Missing and Exploited Children with a picture of a missing child. The officer took a screenshot of the image, which she later fed into a tool created by nonprofit Thorn to help investigators find underage sex-trafficking victims. The tool, called Spotlight, uses text- and image-processing algorithms to match faces and other clues in online sex ads with other evidence.

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