An iPhone app that alerts users about nearby sightings of Immigration and Customs Enforcement (ICE) activity is gaining traction amid President Donald Trump’s immigration crackdown. And it’s proving to be a controversial platform, with ICE speaking out against it on Monday.

Joshua Aaron has worked in and around the tech industry for around two decades. He built his first app — a blackjack game — at computer camp when he was 13.

As protests against ICE raids continue across the United States, a new app is allowing users to report ICE sightings in an instant. ICEBlock helps users gain real-time information on when and where ICE will conduct checks.