On the OK city bombing:

park it in front

The truck was in the parking garage below the building. When it exploded it destroyed the support columns which were keeping the building in the air. With no more support columns the portions of the building supported by them collapsed. This is why most of the building is gone.

The explosive used was homemade from ammonia which was extracted from fertilizer. After the investigation was over recommendations from the FBI were made regarding the process involving purchasing fertilizer with ammonia in it. Namely there was no process at all other than just buying it from a store. The resulting marketplace changed and restrictions regarding concentrations of ammonia, quantity allowed to be sold at a time, and ID requirements for purchasers.

Why create all these restrictions if it wasn't done with ammonia? Other than massively inconveniencing farmers I can't think of an "evil cabal running things" reason for imposing the regulations.