For me the clincher is that McVeigh mixed and loaded the bomb by himself.
Try mixing ONE barrel of dry fertilizer with anything, by hand. I'm pretty fit, and I know how beat I am after mixing just a couple wheelbarrow loads of cement for foundation piers.
It would have taken three people, at least, to make the bomb.
my understanding is that you don't mix, you pour. Fill the container with AN, pour in the Fuel Oil.
/had a class on blasting