Yeah... because all Radio Control Cars and Planes are started by dialing them with your cell phone. (/s)
Even the two damaged circuit boards found are a ruse by the bombers to confuse the investigators.
The ‘fireworks’ these two used are what they called ‘mortar shells’ for an ‘artillery cannon’. They have a LONG FUSE on them, ready to USE. All they had to do was light it, close the pressure cooker, and walk away.
Or use a model rocket spark ignitor hooked to a relay. Only needs about 3v to set ‘em off.
Although they could just light the fuse and walk away, this is not all that easy to get away with in a crowd.
Remote electrical detonation is the way to go.
If they'd used modern electric pressure cookers the timing system would have allowed them to set up about 24 hours in advance of the event!