USA Today http://m.usatoday.com/article/news/2095817
They claimed 270 tons of Ammonium Nitrate. (What kind of ton - the 2000 lb/ton?) 540,000 lbs of Am Nitrate? That would have to be in the two large buildings.
Presuming the round buildings North side were grain storage.
If true, How big a volume is that?
Ammonium nitrate is about 18.6 cubic feet per (American 2000#) ton.
That would be just over 5000 cubic feet, or a 10' X 10' building 50' high.
That's enough volume and weight to go bang.