At the same time or shortly thereafter, Atta-the only terrorist on board trained to fly a jet-would have moved to the cockpit from his business-class seat, possibly accompanied by Omari.
( ( At ( ( the ( same time ) ) ( or ( shortly thereafter ) ) ) ) ( , ( ( ( Atta-the ( only terrorist ) ) ( on ( board ( trained ( to ( fly ( a jet-would ) ) ) ) ) ) ) ( ( have ( ( moved ( to ( the cockpit ) ) ) ( from ( ( ( his ( business-class seat ) ) , ) ( possibly ( accompanied ( by Omari ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN At) (UCP (NP (DT the) (JJ same) (NN time)) (CC or) (ADVP (RB shortly)) (ADVP (RB thereafter)))) (, ,) (NP (NP (JJ Atta-the) (JJ only) (NN terrorist)) (PP (IN on) (NP (NP (NN board)) (VP (VBN trained) (S (VP (TO to) (VP (VB fly) (NP (DT a) (NN jet-would))))))))) (VP (VBP have) (VP (VBN moved) (PP (TO to) (NP (DT the) (NN cockpit))) (PP (IN from) (NP (NP (PRP$ his) (ADJP (JJ business-class)) (NN seat)) (, ,) (VP (ADVP (RB possibly)) (VBN accompanied) (PP (IN by) (NP (NNP Omari)))))))) (. .)))
Atta left his seat in business-class to go to the cockpit.
( Atta ( ( ( ( left ( his seat ) ) ( in business-class ) ) ( to ( go ( to ( the cockpit ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Atta)) (VP (VBD left) (NP (PRP$ his) (NN seat)) (PP (IN in) (ADJP (JJ business-class))) (S (VP (TO to) (VP (VB go) (PP (TO to) (NP (DT the) (NN cockpit))))))) (. .)))