[Tacis]

Duh! This has been clear from the outset! The FBI was expert in infiltrating militias and white/black power groups. They had at least one agent/contact in the group and knew when the bomb would go off. Something happened (I believe that the FBI showboats from Washington were late - it was Willie's administration, after all) and the blast was not stopped.

The ATF weapons stash was not built into the equation and, instead a some good pictures of a destroyed truck, the whole building went up. From there on it was pure cover up. McVeigh kept his mouth shut out of loyalty to guys he didn't know were informing on him and the FBI hustled him to kingdom come. I've always maintained that the Dallas AIC (now retired) has boxes full of CYA documents that would show the whole truth. It does demonstrate how easy it is to use a federal investigation to cover a conspiracy.

[TigerTale]

The ATF weapons stash was not built into the equation

I had not thought of this. Shouldn't there be records somewhere regarding how much of an arsenal was stored in the Murrah Building?