As United 93 left Newark, the flight's crew members were unaware of the hijacking of American 11.
( ( As ( United ( 93 ( left Newark ) ) ) ) ( , ( ( ( the ( flight 's ) ) ( crew members ) ) ( ( were ( unaware ( of ( ( the hijacking ) ( of ( American 11 ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN As) (NP (NNP United) (CD 93) (NN left) (NNP Newark))) (, ,) (NP (NP (DT the) (NN flight) (POS 's)) (NN crew) (NNS members)) (VP (VBD were) (ADJP (JJ unaware) (PP (IN of) (NP (NP (DT the) (NN hijacking)) (PP (IN of) (NP (NNP American) (CD 11))))))) (. .)))
As the flight United 93 left Newark the crew members were tragically unaware of the hijacking of American 11 and if they had been they could have prevented their own hijacking by keeping the plane grounded and double-checking for safety. 
( ( As ( the flight ) ) ( ( ( United 93 ) ( ( left Newark ) ( ( ( ( the ( crew members ) ) ( ( were tragically ) ( unaware ( of ( ( the hijacking ) ( of ( American 11 ) ) ) ) ) ) ) and ) ( if ( they ( had been ) ) ) ) ) ) ( they ( ( could ( have ( ( prevented ( their ( own hijacking ) ) ) ( by ( ( ( keeping ( ( the plane ) grounded ) ) and ) ( double-checking ( for safety ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN As) (NP (DT the) (NN flight))) (PRN (NP (NNP United) (CD 93)) (VP (VBD left) (NP (NNP Newark)) (SBAR (SBAR (S (NP (DT the) (NN crew) (NNS members)) (VP (VBD were) (ADVP (RB tragically)) (ADJP (JJ unaware) (PP (IN of) (NP (NP (DT the) (NN hijacking)) (PP (IN of) (NP (NNP American) (CD 11))))))))) (CC and) (SBAR (IN if) (S (NP (PRP they)) (VP (VBD had) (VP (VBN been)))))))) (NP (PRP they)) (VP (MD could) (VP (VB have) (VP (VBN prevented) (NP (PRP$ their) (JJ own) (NN hijacking)) (PP (IN by) (S (VP (VP (VBG keeping) (NP (NP (DT the) (NN plane)) (VP (VBN grounded)))) (CC and) (VP (VBG double-checking) (PP (IN for) (NP (NN safety)))))))))) (. .)))