One of the flights that received the warning was United 93.
( ( One ( of ( ( the flights ) ( that ( received ( the warning ) ) ) ) ) ) ( ( was ( United 93 ) ) . ) )
(ROOT (S (NP (NP (CD One)) (PP (IN of) (NP (NP (DT the) (NNS flights)) (SBAR (WHNP (WDT that)) (S (VP (VBD received) (NP (DT the) (NN warning)))))))) (VP (VBD was) (NP (NNP United) (CD 93))) (. .)))
United 93 never received a warning.
( ( United 93 ) ( never ( ( received ( a warning ) ) . ) ) )
(ROOT (S (NP (NNP United) (CD 93)) (ADVP (RB never)) (VP (VBD received) (NP (DT a) (NN warning))) (. .)))