The many missed chances to disrupt the Sept. 11 terrorist attacks spanned years: The hijackers methodically devised and implemented their plot beginning in 1996. One of the hijacked jetliners flew undetected for 36 minutes because of a radar glitch.

_Because of a radar glitch, air traffic controllers didn't realize American Airlines Flight 77 — which took off from Dulles Airport outside Washington — might be hijacked when it mysteriously started veering off course at 8:54 a.m. The plane traveled undetected for a critical 36 minutes.

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