All the hijackers had assigned seats in first class, and they seem to have used them.
( ( ( ( ( ( All ( the hijackers ) ) ( had ( assigned ( seats ( in ( first class ) ) ) ) ) ) , ) and ) ( they ( seem ( to ( have ( used them ) ) ) ) ) ) . )
(ROOT (S (S (NP (PDT All) (DT the) (NNS hijackers)) (VP (VBD had) (VP (VBN assigned) (NP (NP (NNS seats)) (PP (IN in) (NP (JJ first) (NN class))))))) (, ,) (CC and) (S (NP (PRP they)) (VP (VBP seem) (S (VP (TO to) (VP (VB have) (VP (VBN used) (NP (PRP them)))))))) (. .)))
The hijackers could not afford first class and sat in the back. 
( ( The hijackers ) ( ( ( ( ( could not ) ( afford ( first class ) ) ) and ) ( sat ( in ( the back ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NNS hijackers)) (VP (VP (MD could) (RB not) (VP (VB afford) (NP (JJ first) (NN class)))) (CC and) (VP (VBD sat) (PP (IN in) (NP (DT the) (NN back))))) (. .)))