Ok... Pure speculation, but what I would do if I was designing an anti-terrorist plan.
Imagine you have immediate access to all cell phone activity in the country. There are reported reasons to believe that this is true...
ASAP prepaid cellphone lockout in the area, and possibly nationwide or just rich target areas.
Almost immediately thereafter cellphone number of the triggers and what phone called them. (This can be done by simply looking at calls that weren't answered at the right time and right location),
Tracking of location of calling phone, to within a cell tower.
Time tracking of triggered phones and calling phone, within a relatively short period of time. Proximity (time/location) to any other preferably pre-paid phones, this will give an indication to additional devices.
Transmit to authorities continual location of calling phone.
Full time tracking of triggered phones. To give location of origin of bomb making, number of devices, time of plot, testing of devices.
Take down of bomb making site.
Additional features I am sure.
All within hours, max. I am sure that this would have been scripted for almost immediate action. Who knows what the constitutional implications are.