I'm sensitive on this subject. My software was used in the NY Coroner's Office to help track and identify the victims, and the then head of IT had me flown out on the first public London/NY service to help get the effort off the ground. Staff working for the company of which I was then Technical Director were intimately concerned in the effort to identify victims, for more than two years, (including a gentleman called Naeem, a Muslim) and I had several visits myself. Another person who I had introduced to the situation, a UK based independent consultant in my product, was awarded an MBE.
The night after I arrived NYC was eerie and distressing. I passed a fire station (can't quite remember the correct US term) where every single fireman had died. it was surrounded by candles and tributes and was one of the most affecting things I've seen in my life.
I have no doubt about the official narrative for the events of the day, although there is legitimate grounds to question how the terrorists were able to succeed.
Edit: Just thought to check my old passport. It was Sep. 20th I arrived. I actually saw Tony Blair being ferried off the plane in front of us.