Giovanni,
It will take weeks, months or maybe longer before the results of the investigations are made public.
Ammonium nitrate has been the "fuel" for many disasters.
Texas City in 1947(?) with 500 fatalities was traced back to a dockworker smoking a cigarette.
There was another incident in Canada that was even worse.
I have been in one major explosion (propane leak on start up of a new unit that killed 3) and I feel for the residents of this community.
In fact, several decades ago I knew people in West, Texas. Unfortunately, I have lost contact with them over the years. I still stop in at the Czech Stop Store in West for colaches (Czech Pastries) and sausages when passing through.
As is usually the case in major disasters, it is a series of (often unrelated) events that independently would not be a large problem, seem to coalesce into an ugly monster....
I have heard speculation that there was fire that created a BLEVE (boiling liquid expanding vapor explosion) on a vessel/storage tank.