A 20-year-old college student used Snapchat’s gender swap filter to catch a police officer looking to hook up with an underage teen girl, NBC Bay Area reports. The student told NBC Bay Area that he was motivated by a friend, who told him she was sexually assaulted as a child, to catch possible predators on Tinder.

Ethan, who has not given his last name for fear of retaliation, posed as a woman named Esther on Tinder, and eventually caught the attention of Robert Davies, a San Mateo police officer. Davies asked, “Are you down to have some fun tonight?” and the student then moved the chat to a different app to let him know that he was 16. Police say from the text screenshots that this didn’t appear to bother Davies, and the student then chatted with Davies for over 12 hours in order to get more information about him. Ethan took screenshots of the explicit conversation in airplane mode (Snapchat notifies the other user if a screenshot is taken), and showed the messages to Crime Stoppers. Davies was arrested last week, and has been put on paid administrative leave while he faces a charge of contacting a minor to commit a felony.

Snapchat’s gender swap filter has gone viral in the past few months, with users posting hyper-masculine or feminine versions of themselves over various social platforms. Public opinion over the lens has been split, with some saying the lens reinforces transphobia and heteronormative ideals of beauty (the male filter skews toward square jaws and beards, while the female filter defaults to a made-up face with long hair), while others say it has liberated them to explore themselves in a new light. In this particular case of the gender swap filter being used to catch sexual predators, the lens has actually made a positive, real-world impact.