On September 11, 2001, US astronaut Frank L. Culbertson watched the aftermath of the 9/11 attacks from the International Space Station. He wrote up his experience that day and the days following in a letter posted on NASA's site:
I glanced at the World Map on the computer to see where over the world we were and noticed that we were coming southeast out of Canada and would be passing over New England in a few minutes. I zipped around the station until I found a window that would give me a view of NYC and grabbed the nearest camera. It happened to be a video camera, and I was looking south from the window of Michael's cabin.
The smoke seemed to have an odd bloom to it at the base of the column that was streaming south of the city. After reading one of the news articles we just received, I believe we were looking at NY around the time of, or shortly after, the collapse of the second tower. How horrible…
I panned the camera all along the East Coast to the south to see if I could see any other smoke around Washington, or anywhere else, but nothing was visible.
It was pretty difficult to think about work after that, though we had some to do, but on the next orbit we crossed the US further south. All three of us were working one or two cameras to try to get views of New York or Washington. There was haze over Washington, but no specific source could be seen. It all looked incredible from two to three hundred miles away. I can't imagine the tragic scenes on the ground.
"Astronaut Frank Culbertson Letter from September 11, 2001" (Thanks, Ari Pescovitz!)