I was across the river from it when it happened. My dad had been working there late the previous night and he had overslept, which was the only reason why he wasn't at work on time. I was in school. Since I was young I don't remember exactly what was happening at the time, but according to my parents my dad was in the subway when the planes crashed. When he returned to the Twin Towers for work (I believe he also had to do something else at the time), he discovered his office was gone. I'm amazingly thankful that my dad hadn't been working there at the time. If he had, he probably would have died since he worked near the top.
Later that night we lit candles and set them on the river. That was really the only part that stood out to me when it happened.