The pilots knew their mission was to divert aircraft, but did not know that the threat came from hijacked airliners.
( ( The pilots ) ( ( ( ( ( knew ( ( their mission ) ( was ( to ( divert aircraft ) ) ) ) ) , ) but ) ( ( did not ) ( know ( that ( ( the threat ) ( came ( from ( hijacked airliners ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NNS pilots)) (VP (VP (VBD knew) (SBAR (S (NP (PRP$ their) (NN mission)) (VP (VBD was) (S (VP (TO to) (VP (VB divert) (NP (NN aircraft))))))))) (, ,) (CC but) (VP (VBD did) (RB not) (VP (VB know) (SBAR (IN that) (S (NP (DT the) (NN threat)) (VP (VBD came) (PP (IN from) (NP (JJ hijacked) (NNS airliners))))))))) (. .)))
Pilots were unaware that the planes were hijacked.
( Pilots ( ( ( were unaware ) ( that ( ( the planes ) ( were hijacked ) ) ) ) . ) )
(ROOT (S (NP (NNS Pilots)) (VP (VBD were) (ADJP (JJ unaware)) (SBAR (IN that) (S (NP (DT the) (NNS planes)) (VP (VBD were) (VP (VBN hijacked)))))) (. .)))