Or hide the planes in small hangars, barns, storage facilities etc- then fly them very quickly toward the desired city...eject the bomb or whatever. By the time our fighters get there- the deed could well be done. Look at those little planes that flew into Philly and DC airspace...
I suspect that is why we only "spot check" containers that come in on ships. The Feds know that the terrorists will just use what we have here. That don't have to use nukes or dirty bombs to kill alot of people.