https://en.wikipedia.org/wiki/Mobile_phone_tracking
Probably got his phone’s unique IMEI at the FedEx office from a roaming signal and were then able to track based on subsequent roaming signals. Since they had the suspect, his phone, and video confirmation in one place, they could then look for where that IMEI showed up (cell tower roam logs).
Are you saying that the Fed's can get a record of all roaming signals and those phone's ID from the FedEx office at the time he dropped off the package even if he didn't use the phone? And then get info on the individual phones and by means of elimination zero in on the bombers phone?