Horrible and shock, yes, but not intentional. I personally don't think that was the reaction from most when the first plane hit.
When the first plane hit, I thought it was a horrible accident -- never gave terrorism a thought.
Oddly enough my first reaction was that I was seeing some special effects from an upcoming movie. But when the second one hit and then the third at the Pentagon and the plane went down in PA. I pretty much knew. AH HA, we have a pattern hear and things is mighty non coincidental. I bet this was a terrorist act against the US.
Course looking back it might have been just some depressed students who wanted attention.