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))) (. .)))
Three flights in total received warnings.
( ( ( Three flights ) ( in total ) ) ( ( received warnings ) . ) )
(ROOT (S (NP (NP (CD Three) (NNS flights)) (PP (IN in) (NP (NN total)))) (VP (VBD received) (NP (NNS warnings))) (. .)))