@bourbon_warrior: Not to mention McVeigh used ammonium nitrate. Though, this still seems like an accident. It didn't start as an explosion; it just started as a reported fire at the highly-compressed anhydrous ammonia tank that had been refilled today. The firefighters weren't able to get it under control, and it appears that the anhydrous ammonia ignited, leading to the explosion on display.
Usually, a plant like this shouldn't be at risk of an explosion, but something must have gone wrong for that to happen.
The plant owners believed that the worst case scenario was that the ammonia would just vent into the atmosphere, maybe the ventilation system failed or something and the gas didn't get released.