From my understanding the Ammonium Nitrate has to be soaked with fuel to be ignitable, I dont see fuel oil evenly being soaked into the fertilizer unless someone purposefully did so, whether we truly find out who was behind this we'll never know, the evidence was so thoroughly obliterated, it would take a huge team of investigators just from the U.S. to resolve just who did this.
It sure was no nuke, you would have radiation showing and radiation sickness.