Complex sentence: The 9/11 Commission estimated that the flight was hijacked between 08:51 and 08:54, shortly after American Airlines Flight 11 struck the World Trade Center and not too long after United Airlines Flight 175 had just been hijacked.
Split into simpler sentences: The 9/11 Commission estimated that the flight was hijacked between 08:51 and 08:54. By this time, American Airlines Flight 11 had already struck the World Trade Center and United Airlines Flight 175 had just been hijacked.