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))) (. .)))
Flight United 93 received a warning.
( ( Flight ( United 93 ) ) ( ( received ( a warning ) ) . ) )
(ROOT (S (NP (NNP Flight) (NNP United) (CD 93)) (VP (VBD received) (NP (DT a) (NN warning))) (. .)))