When the event happened the stock market was not yet open because stock exchanges in the USA open by 9 30 am, and the first plane hit before then. However, the S&P 500 Futures market was open because Futures markets open on Sunday evening.
This is the sequence of events on the S&P 500 Futures chart
1. First plane hits the tower
2. CNN reports the news
3. FOX news & CBS news report the incident, President Bush is briefed
4. Second plane crashes into the tower