Large amounts of of ammonium nitrate fertilizer are not the worlds smartest choice for open air bombs. The explosion is much too slow to do massive damage, such as OKC in '95. That would be if you could even get an entire truck load to ignite. It needs careful mixing.
I was think along the lines of 200 terrorist in a trailer instead of being packed with 200 illegal aliens, weapons, bio-weapons, dirty bombs or material for other explosives, shoulder fired weapons, and God only knows what else.