And yet no one came forth to call for stopping the execution of Timothy McVeigh. I don't say that he would have ever opened up, but he seemed determined that this was going to be a point of personal challenge with him. He would rather die than reveal his connections, and chose to let the circumstances bear him along. All the evidence seemed to confirm the popular view that he was the only person that drove the truck in and parked it, but his escape path is not fully understood or accounted for, which is the opportunity where this third party (or even other additional parties) may have been providing some very crucial assistance. There is also other anecdotal indications that much more assistance than was first acknowledged was provided in constructing and setting up the fuse devices for the Ammonium Nitrate-Fuel Oil explosive delivered in the rental truck. I understand this is a rather tricky thing, and requires considerable expertise, which may or may not have been provided by certain outside associates.
We are pretty sure that the outside associates were not part of any of the domestic militia groups. For one thing, ANFO would not be the explosive of choice. They would have gone for stolen or trafficked demolition explosives, from commercial or military sources.
McVeigh simply wanted to die and become a martyr.