Because several passengers on United 93 described three hijackers on the plane, not four, some have wondered whether one of the hijackers had been able to use the cockpit jump seat from the outset of the flight.
( ( Because ( ( ( several passengers ) ( on ( United 93 ) ) ) ( ( ( ( described ( three hijackers ) ) ( on ( the plane ) ) ) , ) ( not four ) ) ) ) ( , ( some ( ( have ( wondered ( whether ( ( one ( of ( the hijackers ) ) ) ( had ( been ( able ( to ( ( use ( the ( cockpit ( jump seat ) ) ) ) ( from ( ( the outset ) ( of ( the flight ) ) ) ) ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN Because) (S (NP (NP (JJ several) (NNS passengers)) (PP (IN on) (NP (NNP United) (CD 93)))) (VP (VBD described) (NP (CD three) (NNS hijackers)) (PP (IN on) (NP (DT the) (NN plane))) (, ,) (ADVP (RB not) (NP (CD four)))))) (, ,) (NP (DT some)) (VP (VBP have) (VP (VBD wondered) (SBAR (IN whether) (S (NP (NP (CD one)) (PP (IN of) (NP (DT the) (NNS hijackers)))) (VP (VBD had) (VP (VBN been) (ADJP (JJ able) (S (VP (TO to) (VP (VB use) (NP (DT the) (NN cockpit) (NN jump) (NN seat)) (PP (IN from) (NP (NP (DT the) (NN outset)) (PP (IN of) (NP (DT the) (NN flight))))))))))))))) (. .)))
There were exactly three hijackers on the plane.
( There ( ( ( were exactly ) ( ( three hijackers ) ( on ( the plane ) ) ) ) . ) )
(ROOT (S (NP (EX There)) (VP (VBD were) (ADVP (RB exactly)) (NP (NP (CD three) (NNS hijackers)) (PP (IN on) (NP (DT the) (NN plane))))) (. .)))