You can do a lot of tracking without cell phone data.
Local police have access to license plate scanners (both at fixed locations and mounted on police vehicles). The data is not as fine-grained as GPS data from a person's phone, but it can also yield information about the patterns of a person's movements over time (e.g., they pass these ten intersections every weekend night between 2:30 and 3:00am). Of course, this can only track people with cars (but in some cities, that's pretty much everyone).
Facial recognition software can track people who walk, but I don't know how widely deployed it is at this time. As soon as it gets attached to all of the millions of surveillance cameras that are out there (some of which are operated by police departments, like in NYC), tracking pedestrians will become easier.
And, of course, the local police can get a warrant to attach a GPS device to somebody's car or to pull cell tower data if they have probable cause.
Comments
You can do a lot of tracking without cell phone data.
Local police have access to license plate scanners (both at fixed locations and mounted on police vehicles). The data is not as fine-grained as GPS data from a person's phone, but it can also yield information about the patterns of a person's movements over time (e.g., they pass these ten intersections every weekend night between 2:30 and 3:00am). Of course, this can only track people with cars (but in some cities, that's pretty much everyone).
Facial recognition software can track people who walk, but I don't know how widely deployed it is at this time. As soon as it gets attached to all of the millions of surveillance cameras that are out there (some of which are operated by police departments, like in NYC), tracking pedestrians will become easier.
And, of course, the local police can get a warrant to attach a GPS device to somebody's car or to pull cell tower data if they have probable cause.