(S (S (S (NP (BASENP (NNP NATO)) (CC and) (BASENP (JJ Afghan) (NNS authorities))) (VP (VBP are) (VP (VBG investigating)))) (CC but) (S (BASENP (NNP NATO) (NNS officials)) (VP (VBP have) (VP (ADVP (RB yet)) (S (VP (TO to) (VP (VB comment)))))))) (PERIOD .))