I would have thought quite unlikely on September 10th.
I still remember my first reaction when Carol told me "a plane flew into the WTC in New York.":
"What? You're kidding? No, you're not! You wouldn't kid about something like that!"
Then when the second plane flew into the other tower:
"What's wrong with the world?"
I've heard that answer before. It would seem that the technical difficulties in hijacking planes are not as complex as the technical difficulties of smuggling nuclear weapons into the US and detonating them, at least to me...