All speculation at this point.
My first guess was TATP but they could have hit multiple fireworks stores and tried to keep their purchases under the radar.
BATF/FBI will eventually issue a statement on what was used. It probably will point to the fireworks as a source even if it was high grade C4.
What about tannerite (sp)? This has been in the news recently here in Indiana. Seems to be pretty potent and readily available here.
My guess was tatp too. It was a very fast explosive that burns clean. The lack of residue in the recovered components points to a high speed explosive. It was not black powder but something much more powerful.