As it began, some of the hijackers-most likely Wail al Shehri and Waleed al Shehri, who were seated in row 2 in first class-stabbed the two unarmed flight attendants who would have been preparing for cabin service.
( ( As ( it began ) ) ( , ( ( ( ( some ( of ( the ( ( hijackers-most likely ) ( Wail ( al Shehri ) ) ) ) ) ) and ) ( ( ( Waleed ( al Shehri ) ) , ) ( who ( were ( seated ( in ( ( row 2 ) ( in first ) ) ) ) ) ) ) ) ( ( class-stabbed ( ( the ( two ( unarmed ( flight attendants ) ) ) ) ( who ( would ( have ( been ( preparing ( for ( cabin service ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN As) (S (NP (PRP it)) (VP (VBD began)))) (, ,) (NP (NP (NP (DT some)) (PP (IN of) (NP (DT the) (ADJP (JJ hijackers-most) (JJ likely)) (NNP Wail) (NNP al) (NNP Shehri)))) (CC and) (NP (NP (NNP Waleed) (NNP al) (NNP Shehri)) (, ,) (SBAR (WHNP (WP who)) (S (VP (VBD were) (VP (VBN seated) (PP (IN in) (NP (NP (NN row) (CD 2)) (PP (IN in) (NP (JJ first))))))))))) (VP (VBD class-stabbed) (NP (NP (DT the) (CD two) (JJ unarmed) (NN flight) (NNS attendants)) (SBAR (WHNP (WP who)) (S (VP (MD would) (VP (VB have) (VP (VBN been) (VP (VBG preparing) (PP (IN for) (NP (NN cabin) (NN service))))))))))) (. .)))
The hijackers were planning on attacking the nation.
( ( The hijackers ) ( ( were ( planning ( on ( attacking ( the nation ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NNS hijackers)) (VP (VBD were) (VP (VBG planning) (PP (IN on) (S (VP (VBG attacking) (NP (DT the) (NN nation))))))) (. .)))