Human traffickers commonly advertise children for sex using explicit pictures taken from inside hotel rooms. But now a new app, the TraffickCam, enables travelers to help law enforcement track down traffickers by submitting pictures of hotel rooms around the world that can be matched against a national database used by police.

The idea for the app is originated with researchers at Washington University and the Exchange Initiative, a non-profit formed by Nix Conference and Meeting Management. after police sought the help of Nix staff to identify the specific hotel where a victim was trafficked. “It was a photo that they had from the internet,” Nix Principal Molly Hackett said. “One of the girls in our office knew exactly what it was.”

TraffickCam has built a database of more than 1.5 million images of hotel rooms across the world that police can query to determine where pictures of trafficking victims might have been taken, thanks to photographs submitted by travelers. The police can upload an image to the app and compare it against photographs uploaded by the traveling public, then use that information to narrow down a list of possible locations where the trafficker took the picture of a child being sold for sex.

“You just enter your hotel room, and your room number. You take four pictures, and you submit them to the website,” Washington University Researcher and TraffickCam developer Abby Stylianou said. “And then those become part of the pipeline that law enforcement can use to track down where the victims are being trafficked.”

Carol Smolenski, executive director of ECPAT-USA, the American offshoot of an international nonprofit dedicated to ending sexual exploitation of children, said kids are often bought and sold in hotel rooms from high-end to budget motels.

“The takeaway is that it’s a phenomenon in the United States, and most people don’t think that’s true. They think it’s happening somewhere else,” she said. “The whole industry is off the streets, so those transactions have to happen someplace. Very frequently it’s in a hotel room.”

The app was launched on 20 June 2016 and already has been downloaded more than 56,000 times. Hackett estimated that it’s receiving around 1,700 new submissions a day from hotel-goers.