A MARTÍNEZ, HOST: Federal immigration officers often use facial recognition in the field to identify undocumented immigrants. Now, a newly revealed document from the Department of Homeland Security outlines plans to give local police the same technology. NPR's Meg Anderson is here to explain this. So, Meg, tell us more about the DHS document and the tech it describes. MEG ANDERSON, BYLINE: Yeah. So the existence of this particular document was first reported by the news site 404 Media. And it describes a mobile app designed for local police who are deputized by ICE to enforce immigration law. Police use the app to scan the faces of people they stop. The app then runs those faces against more than 250 million government records. We're talking State Department visa records, the database TSA uses at airports. Once someone's face is scanned, the app tells police whether or not that person can be detained for an immigration violation. MARTÍNEZ: All right. So what do we know about how police will actually use this technology? ANDERSON: Not a lot, but we do know the app launched last September. That suggests that police have already been using it. We don't know for sure. And then there's a question of whether police need to have some reason or suspicion to stop people before they use the app. I spoke to Clare Garvie about that. She's a lawyer with the Policing Project at New York University. CLARE GARVIE: Can they walk around taking photos of whoever as sort of a dragnet way to attempt to identify individuals who might be in the country unlawfully? ANDERSON: And I should note, ICE and Border Patrol already use facial recognition technology. During ICE surges in various cities, it's been common for federal officers to take photos of people, both