At this point someone needs to run an SDR and start capturing as much RF spectrum as possible, especially on any communications device that has a 'selective calling' feature.

The pagers could have been set off with a page sent to a 'group' capcode in a hidden slot with a unique beep pattern that a little tiny MCU picked up and set off the detonator.

Radios -- same thing. Possibly a group calling feature of a signalling system was used with a "secret" group hidden away in the radio programming?