National Fire Protection Code 490 on the Storage of Ammonium Nitrate.
NFPA 490: Code for the Storage of Ammonium Nitrate
http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=490
Looks to me like the center of the blast location was in the North West corner of the large building. Or possibly right outside where the Trailer was parked in the Google picture.
We don’t know if there was a trailer there that day. But we do know that they park them in that location.
Implies a fork lift operation lifting pallets of fertilizer out of a trailer and into their warehouse.