It is impossible that the destruction to the building could have resulted from such a bomb alone. To cause the damage pattern that occurred to the Murrah building, there would have to have been demolition charges at several supporting column bases, at locations not accessible from the street, to supplement the truck bomb damage. Indeed, a careful examination of photographs showing the collapsed column bases reveals a failure mode produced by demolition charges and not by a blast from the truck bomb. To understand what caused the damage to the Murrah Building, one needs to understand some basics about the use and nature of explosives.
Physics911, by Scientific Panel Investigating Nine-Eleven, 9/11/2001