Taylor Swift’s concert at the Rose Bowl in Los Angeles, California revealed that it used facial recognition to seek out the popstar’s hundreds of stalkers.

Usually, Swift’s concerts include an array of exciting elements; during her shows on the Reputation Tour, staff handed out light-up bracelets that coincided with her music. So, when fans stepped up to watch rehearsal clips at a special koisk at the May 18 concert in LA, they didn’t think anything of it. However, it turns out, that kiosk actually had a camera hidden inside that took photographs of fans.

The system used these images to cross-reference pictures in a Nashville database that included all of Swift’s known stalkers, Rolling Stone reported. The chief security officer of Oak View Group, which is an advisory board for multiple concert venues, noted that “everybody who went by would stop and stare at it, and the software would start working.”

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While this raises privacy concerns, since it is unknown where this information goes or where it is stored, Swift does not have an obligation to tell attendees that they are being surveilled, since her concerts are technically private events. While some fans are upset about being watched without warning, the system is in place for Swift’s safety.

Just in 2018, Swift had to get two restraining orders against men that were threatening her. In May, a man was arrested outside of her Beverly Hills home who wore a mask and had a knife in his car, and another was sentenced to six months in jail for appearing at Swift’s New York home five times within two months. Then, in September, Swift had to file a restraining order against a man who had been harassing her since 2016 with rape and murder threats.

This isn’t the first time facial recognition has been considered at concerts. Earlier this year, Ticketmaster announced that it would be investing in the facial recognition company called Blink Identity, which means that concertgoers could just use their face as their ticket to enter a show. However, this may be offered to high rollers and VIP guests before the general public.

Ticketmaster’s chief product officer, Justin Burleigh, told Rolling Stone that the company plans to beta-test the technology at venues early next year.

“It holds a lot of promise,” he said. “We’re just being very careful about where and how we implement it.”