Was the FBI Behind the Oklahoma Bombing?

It has always been hard to believe that the truck-bombing of the A.P. Murrah Federal Building in Oklahoma City on April 19, 1995, which killed 168 people, including 19 children at a daycare center, was planned by just one or two perpetrators acting alone. However, the official story states that the mastermind was Gulf War veteran Timothy McVeigh, and that the two others sentenced with him only helped him in various ways.