I was working for a company in Winchester, Hampshire with American clients, as well as pretty much every other country. My Managing Director was upstairs in a meeting with a top financial firm and on the phone they had some US colleagues, who were inside the Twin Towers at the time, luckily for them on the ground floor, they got out. I had emails from my parents, best friends pop on my PC, saying watch the BBC news, something fucking terrible is happening in New York, a plane has crashed into the WTC. WTF? By this point everyone in our office were gathering in the middle of the room, looking shell shocked, everyone was desperately trying to get onto the news site but the server had crashed. Our MD came downstairs with the customers in tow, he had tears in his eyes, at the time I didn't know who the guests were nor the fact they had colleagues in the impacted building, that all came to light in the days that followed.
The company I worked for produced software for the Stock market industry, I think the big client were PWC.
The industry took a big hit at the time and as a result a lot of people lost their jobs in the wake of the attacks, including myself and that was the catalyst for me going conracting for twenty years or so, until Covid forced me to close it all down and return to a normal salaried job.
A good friend of ours visited Ground Zero last year when she flew over to New York.