At 9:03, United 175 crashed into the South Tower.
( ( At 9:03 ) ( , ( ( United 175 ) ( ( crashed ( into ( the ( South Tower ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN At) (NP (CD 9:03))) (, ,) (NP (NNP United) (CD 175)) (VP (VBD crashed) (PP (IN into) (NP (DT the) (NNP South) (NNP Tower)))) (. .)))
When United 175 crashed, the South Tower fell to the ground.
( ( When ( ( United 175 ) crashed ) ) ( , ( ( the ( South Tower ) ) ( ( fell ( to ( the ground ) ) ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NNP United) (CD 175)) (VP (VBD crashed)))) (, ,) (NP (DT the) (NNP South) (NNP Tower)) (VP (VBD fell) (PP (TO to) (NP (DT the) (NN ground)))) (. .)))