(S (S (BASENP (PRP It)) (VP (VP (VBZ is) (RB not)) (VP (ADVP (RB yet)) (VP (VBN known) (SBAR (WHNP (WP who)) (S (VP (VBD was) (ADJP (JJ responsible) (PP (IN for) (NP (BASENP (DT the) (NN attack)) (PP (IN on) (BASENP (DT the) (NNP UN) (NN convoy))))))))))))) (PERIOD .))