The threat of terrorists hijacking commercial airliners within the United States-and using them as guided missiles-was not recognized by NORAD before 9/11.
( ( ( The threat ) ( of ( terrorists ( ( hijacking ( commercial airliners ) ) ( within ( ( the ( United States-and ) ) ( ( using them ) ( as ( guided missiles-was ) ) ) ) ) ) ) ) ) ( ( not ( recognized ( by ( NORAD ( before 9/11 ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN threat)) (PP (IN of) (NP (NP (NNS terrorists)) (VP (VBG hijacking) (NP (JJ commercial) (NNS airliners)) (PP (IN within) (S (NP (DT the) (NNP United) (NNP States-and)) (VP (VBG using) (NP (PRP them)) (PP (IN as) (NP (JJ guided) (NNS missiles-was)))))))))) (VP (RB not) (VBN recognized) (PP (IN by) (NP (NP (NNP NORAD)) (PP (IN before) (NP (CD 9/11)))))) (. .)))
NORAD didn't recognize that airplanes could pose a serious threat,
( NORAD ( ( ( did n't ) ( recognize ( that ( airplanes ( could ( pose ( a ( serious threat ) ) ) ) ) ) ) ) , ) )
(ROOT (S (NP (NNP NORAD)) (VP (VBD did) (RB n't) (VP (VB recognize) (SBAR (IN that) (S (NP (NNS airplanes)) (VP (MD could) (VP (VB pose) (NP (DT a) (JJ serious) (NN threat)))))))) (, ,)))