At least one other person (a "person of interest" at the rental car counter). Possibly some foreign assistance or training as well. Definite contacts in the Philippines.
The US government agencies monitoring the group waited too long before rolling them up. They also got played by their informant.
Additional bonus. The blast cooked off military munitions that were never supposed to be in the building in the first place. How convenient that all the neighboring storefront surveillance videos were impounded and cannot now be located. Multiple explosions on site, readily detected by USGS seismic monitors in the State.
Massive RIFs at USGS afterwards too. The folks who mentioned the seismic recordings are gone. So are the recordings. Just a coincidence, you can be sure.
A huge cluster%^&^%^. Promotions and reassignments for everybody. Just keep it quiet. Or else.
This is raw rumor. No verification available. Make of it what you will.
Believable though!