Complex sentence: The order to dispatch the fighters at Otis was given at 08:46, and the F-15s took off at 08:53, roughly seven minutes after American Airlines Flight 11 had already crashed into the North Tower of the World Trade Center.
Split into simpler sentences: The order to dispatch the fighters at Otis was given at 08:46, and the F-15s took off at 08:53. By that time, American Airlines Flight 11 had already crashed into the North Tower of the World Trade Center.