A smoldering gun
Not quite the "smoking gun" that is needed. Unfortunately, unless more definitive evidence is derived from further analysis we will be left simply speculating on the purpose of these vans.
Of course what's missing yet from the Bush list are the following:
* 500 tons of mustard gas and nerve gas
* 25,000 liters of anthrax
* 38,000 liters of botulinum toxin
* 29,984 prohibited munitions capable of delivering chemical agents
* Several dozen Scud missiles
* Gas centrifuges to enrich uranium
* Long-range unmanned aerial vehicles to dispense anthrax