(S (S (S (BASENP (DT The) (NNS helicopters)) (VP (VBD were) (VP (VBN believed) (S (VP (TO to) (VP (VB have) (VP (VBN been) (VP (VP (VBG escorting) (BASENP (DT a) (NNP U.S.) (JJ military) (NN convoy))) (PP (IN on) (BASENP (DT the) (NN ground))))))))))) (SBAR (WHADVP (WRB when)) (S (BASENP (NNS shots)) (VP (VBD were) (VP (VBN fired)))))) (PERIOD .))