"Besides as has been said many times before, the hijack an airliner and fly it into a building scenario is pretty much of a non runner these days. The terrorists have moved on. I'm sure something equally diabolic is being plotted somewhere. The gun thing is a red herring frankly.
"
Corsair,
A simple observation would be that the use of truck bombs has not seemed to decline after years of use. Easier to carry off than an air attack, but still popular.
Intel has shown that repeated probes have been made since 9/11. This of course discounts the majority of events where hysterics were involved.