Steal an armored van for bomb delivery?
I believer Ryder is the terrorist's truck of choice. They've certainly been good performers for them. Seriously though I can't imagine what advantage an aromored van or truck would have for bomb delivery. The things don't have all that much load carrying capacity, since they are pretty weighed down with the armor and such. I don't know, maybe an armored truck would shield radiation from the contents better, although I doubt the differance if any would be enough to bother with. OTOH, the tow truck itself could have use in towing something heavy, as a low tech device might be. I guess you could build a daisy cutter (nothing high tech about them, they're just heavy) what appeared to be something a tow truck might be hauling, and then drop it off somewhere you don't like, and hope the timer works better than the one McViegh was involved with.
Heck, give them a ticket to Gitmo until it's determined exactly who they are.
Maybe these guys were just trying to penetrate security to see if they could or to see what the response would be. This sounds like a scouting mission to me. Maybe they figured that if they drove a prohibited vehicle (but clean of anything incriminating) into the security zone, they would just be told to leave and/or pay a ticket on a fairly bogus charge, the payoff being that they get a firsthand look at the security hardware and procedures.