By 9:20, Indianapolis Center learned that there were other hijacked aircraft, and began to doubt its initial assumption that American 77 had crashed.
( ( By 9:20 ) ( , ( ( Indianapolis Center ) ( ( ( ( ( learned ( that ( there ( were ( other ( hijacked aircraft ) ) ) ) ) ) , ) and ) ( began ( to ( ( doubt ( its ( initial assumption ) ) ) ( that ( ( American 77 ) ( had crashed ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN By) (NP (CD 9:20))) (, ,) (NP (NNP Indianapolis) (NNP Center)) (VP (VP (VBD learned) (SBAR (IN that) (S (NP (EX there)) (VP (VBD were) (NP (JJ other) (JJ hijacked) (NN aircraft)))))) (, ,) (CC and) (VP (VBD began) (S (VP (TO to) (VP (VB doubt) (NP (PRP$ its) (JJ initial) (NN assumption)) (SBAR (IN that) (S (NP (NNP American) (CD 77)) (VP (VBD had) (VP (VBD crashed)))))))))) (. .)))
People began thinking that American 77 was the plane which was hijacked.
( People ( ( began ( thinking ( that ( ( American 77 ) ( was ( ( the plane ) ( which ( was hijacked ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNS People)) (VP (VBD began) (S (VP (VBG thinking) (SBAR (IN that) (S (NP (NNP American) (CD 77)) (VP (VBD was) (NP (NP (DT the) (NN plane)) (SBAR (WHNP (WDT which)) (S (VP (VBD was) (VP (VBN hijacked)))))))))))) (. .)))