(S (S (NP (NP (NP (BASENP (DT A) (NN collision)) (PP (IN between) (BASENP (NN two) (NNP United) (NNPS States) (NN military) (NNS helicopters)))) (PP (IN in) (BASENP (NNP Iraq)))) (SBAR (WHNP (WDT that)) (S (VP (VP (VBD killed) (BASENP (NN four))) (BASENP (JJ last) (NN month)))))) (VP (VBZ has) (VP (VBN been) (VP (VBN blamed) (PP (IN on) (BASENP (NN enemy) (NN fire))))))) (PERIOD .))