All five hijackers passed through the Main Terminal's west security screening checkpoint; United Airlines, which was the responsible air carrier, had contracted out the work to Argenbright Security.
( ( ( ( ( All ( five hijackers ) ) ( passed ( through ( ( the ( Main ( Terminal 's ) ) ) ( west ( security ( screening checkpoint ) ) ) ) ) ) ) ; ) ( ( ( ( ( United Airlines ) , ) ( which ( was ( the ( responsible ( air carrier ) ) ) ) ) ) , ) ( had ( ( ( contracted out ) ( the work ) ) ( to ( Argenbright Security ) ) ) ) ) ) . )
(ROOT (S (S (NP (DT All) (CD five) (NNS hijackers)) (VP (VBD passed) (PP (IN through) (NP (NP (DT the) (NNP Main) (NNP Terminal) (POS 's)) (JJ west) (NN security) (NN screening) (NN checkpoint))))) (: ;) (S (NP (NP (NNP United) (NNPS Airlines)) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBD was) (NP (DT the) (ADJP (JJ responsible)) (NN air) (NN carrier))))) (, ,)) (VP (VBD had) (VP (VBN contracted) (PRT (RP out)) (NP (DT the) (NN work)) (PP (TO to) (NP (NNP Argenbright) (NNP Security)))))) (. .)))
United Airlines had been contracting out work to Argenbright Security for the past five years.
( ( United Airlines ) ( ( had ( been ( ( ( contracting out ) work ) ( to ( ( Argenbright Security ) ( for ( the ( past ( five years ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP United) (NNPS Airlines)) (VP (VBD had) (VP (VBN been) (VP (VBG contracting) (PRT (RP out)) (NP (NN work)) (PP (TO to) (NP (NP (NNP Argenbright) (NNP Security)) (PP (IN for) (NP (DT the) (JJ past) (CD five) (NNS years)))))))) (. .)))