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 ignored evidence that terrorists intended to use airliners as weapons.
( NORAD ( ( ( ignored evidence ) ( that ( terrorists ( intended ( to ( ( use airliners ) ( as weapons ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP NORAD)) (VP (VBD ignored) (NP (NN evidence)) (SBAR (IN that) (S (NP (NNS terrorists)) (VP (VBD intended) (S (VP (TO to) (VP (VB use) (NP (NNS airliners)) (PP (IN as) (NP (NNS weapons)))))))))) (. .)))