Here's a thought for all. The bomb in the truck was not strong enough to create a crater so it was not as strong as some would have you think. But it may have been strong enough to se off explosives already in the building.
And a report generated by retired General Benton Partin (US Air Force) who was chief or amrs development clearly shows columns close to the "blast" untouched while columns further away where blown away. The burn patterns (lack of near) the truck and more intense further away clearly indicates there was more than one bomb. The way the columns were sheared implies the use of cutter charges rather than being pushed down by a blast. Google Benton Partin and you'll be swayed if you arn't already.