It premiered right after (Nov 6) so it's probably safe to say it was at least written, filmed, and produced mostly pre-9/11.

So I think I'm, at best, only partly right.

Wikipedia says the pilot was filmed in March 2001, and production began in July 2001, so the broad strokes of the show were maybe mostly written prior to 9/11, but most of the actual filming likely happened after (which means writers also had time to rewrite at least some things).

I find it interesting that 24 format, total chronological order, allowed them to react to that 9/11 if it was required. Kinda like South Park episodes are at most 2-week old when aired. South Park it's easy since episodes aren't connected and due to how it's made, but the idea is the same.