Location Transport

How would the FBI track a cell phone?

I'm not on the run or anything, I'm just writing a story and want to get the facts straight before sounding like a fool. How exactly would the FBI trace a call from a cell phone. For example, from the story, a girl is being held captive and whatnot by two guys and she knocks one of them over the head and takes his cell phone and calls a FBI agent that had given her their number. If she leaves the call going, can the FBI agent get someone to trace the call back to the girls location or would the girl have to be on the phone WITH the person that tracks it? Does that make sense?

Public Comments

1. They would triangulate the phone's position using the built in GPS chip. If I'm not mistaken, they would need a short amount of time to do get a satellite signal on the phone because it's a different satellite than the phone function.
2. A cell phone's location can be traced whether or not you're making a call. In fact, its location can be traced after the fact (so the FBI could find out where the phone was a few hours ago), although this doesn't go back indefinitely. There's no requirement that the girl be on the phone to do this. The police have been known to find evidence by seeing where an offender's phone was when a crime took place. The police in Australia once used this technique to find where the offender had gone after the crime, and found the murder weapon from this information.
3. If the girl called 911 and the 911 call center's GPS software is working, the GPS device on the phone will tell the 911 call center her location, if the phone is receiving GPS signals. If not, the FBI would only be able to trace it to the cell tower near the phone. If the girl called the FBI without calling 911, then it would take longer, a lot longer. What you see on TV where they trace calls in minutes is not real.