I was working at a Port of Entry as a contractor...the instinct of those supervisors on the ground was to close the border. In fact some smaller border crossings were briefly closed. Common sense. Then word came down from above, not to close the border. That seems like a meaningless blip, of course legal crossings would at some point need to open, but the order was not to close them at all.
It did strike me as very odd, and still does that the official word was not to close even temporarily, even while we were under attack and so much was unknown.
We should have shut the border crossings, increased security and then reopened them. We should have secured the entire border ASAP, as a security priority. It has always been a huge question in my mind why we didn’t.
Another memory, related to that subject:
The day after the attack, September 12th, the government of China announced a list of fourteen countries (IIRC) who's passport holders would not be allowed to fly on any airlines owned by China, the largest being Air China.
The announcement stated that they would be reimbursed for the price of their tickets, and that China apologized for any inconvenience caused by this decision.
All countries on the list were muslim countries.
That was it. No ifs, ands, or buts. End of story.