I would guess that the explosive used contained very little if any gun powder because the ‘smoke’ was so white. This is from stuff a person can get as a fertilizer even possibly a drug store, no gun powder as such involved. Just goes to show how ignorant people on a crusade against guns will/can twist things.
You have obviously never seen black powder go off. The smoke produced is very white indeed, as is the case with black powder substitutes(at least the ones I have used). It is easy to make black powder, any one can do it as long as the ingredients are available and they are. Taggents would not pin point a buyer who paid cash or made his own. This was debated heavily when the push was on to introduce taggents to explosives. BTW, gun powders, including the black powder used in muzzle loaders, are not classified as explosives, they are propellents.
One more fact, the expression gun powder is usually used when referring to black powder, not smokeless.