Certainly the men who hijacked the various planes used in those attacks were planning illegal actions, but I was under the impression that they did have valid visas at the time that they boarded their respective flights.
I’m just sick of people using the fact that the people involved with 9/11 were aliens as a reason to propose some rather draconian measures (such as repealing the 14th amendment) to combat illegal immigration. I could do a Google search, I know, and likely get the answer I need, but frankly, I know my fellow Dopers are just as likely to give me a better answer than Google would offer, and also share some of their thoughts on the same issue.
Controlling the borders is important, but implying that illegal immigrants are the cause of such actions as 9/11 is contrary to what I recall of the event.
I did a little more looking on this–apparently there are some who dispute the government’s version; there may have been some falsification of visas or other shennanigans. It seems universally agreed, however, that none of the hijackers slipped across the southern border in a panel truck
You also sometimes see the false statement floating about that the hijackers entered the US from Canada. If you need help debunking that statement, check out this thread: Need help debunking a 9/11 myth.