That nine minutes' notice before impact was the most the military would receive of any of the four hijackings.
( ( ( ( That ( nine ( minutes ' ) ) ) notice ) ( before impact ) ) ( ( was ( ( the most ) ( ( the military ) ( would ( receive ( of ( any ( of ( the ( four hijackings ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (NP (DT That) (CD nine) (NNS minutes) (POS ')) (NN notice)) (PP (IN before) (NP (NN impact)))) (VP (VBD was) (NP (NP (DT the) (JJS most)) (SBAR (S (NP (DT the) (NN military)) (VP (MD would) (VP (VB receive) (PP (IN of) (NP (NP (DT any)) (PP (IN of) (NP (DT the) (CD four) (NNS hijackings))))))))))) (. .)))
The military had  notice of the hijackings hours before they impacted.
( ( The military ) ( ( ( had ( notice ( of ( the ( hijackings hours ) ) ) ) ) ( before ( they impacted ) ) ) . ) )
(ROOT (S (NP (DT The) (NN military)) (VP (VBD had) (NP (NP (NN notice)) (PP (IN of) (NP (DT the) (JJ hijackings) (NNS hours)))) (SBAR (IN before) (S (NP (PRP they)) (VP (VBD impacted))))) (. .)))