It is appropriate, therefore, to reconsider whether United 93 would have been intercepted.
( It ( ( ( ( ( ( is appropriate ) , ) therefore ) , ) ( to ( reconsider ( whether ( ( United 93 ) ( would ( have ( been intercepted ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (ADJP (JJ appropriate)) (, ,) (ADVP (RB therefore)) (, ,) (S (VP (TO to) (VP (VB reconsider) (SBAR (IN whether) (S (NP (NNP United) (CD 93)) (VP (MD would) (VP (VB have) (VP (VBN been) (VP (VBN intercepted))))))))))) (. .)))
United 93 definitely could not be intercepted.  
( ( United 93 ) ( definitely ( ( ( could not ) ( be intercepted ) ) . ) ) )
(ROOT (S (NP (NNP United) (CD 93)) (ADVP (RB definitely)) (VP (MD could) (RB not) (VP (VB be) (VP (VBN intercepted)))) (. .)))