How? They use a shoebox-sized device called a StingRay. This device (also called an IMSI catcher) mimics cell phone towers, prompting all the phones in the area to connect to it even if the phones aren’t in use.
The police use StingRays to track down and implicate perpetrators of mainly domestic crimes. The devices can be mounted in vehicles, drones, helicopters, and airplanes, allowing police to gain highly specific information on the location of any particular phone, down to a particular apartment complex or hotel room.