I think prior to 9/11 there was always a pent up Xenophobia in this country among certain people. The attacks caused some people to rationalize and verbalize their fears which are born of ignorance. I remember right after 9/11 I was at a bar having a beer when a couple of ladies were having a heated discussion where they attacked all things foreign. They were using 9/11 as a reason for their anger. In their opinion we should cease all immigration legal or illegal so that things like this would never happen again. Of course I had to throw some gasoline on their fire and set them straight. Their anger honestly boiled down to the fact that they and their friends had trouble finding well paying jobs, probably because they lacked skills, but nevertheless they blamed it on foreigners. These ladies were of the trash variety, I'll let you guess what color. Hint Snow _____ and the Seven Dwarves.
I infuriated them when I told them that I have worked and gone to school with with many foreign born US citizens including Muslims. Of course they hit the roof when they heard that. One of them went to get the owner to throw me out, but he was passed out in his office. In conclusion, I don't think 9/11 changed peoples opinions so much as it gave a significant number of people the rationale to verbalize and act on their deeply rooted prejudices.