(S (S (NP (BASENP (NN Security) (NNS officials)) (PRN (COMMA ,) (ADVP (RB however)) (COMMA ,))) (VP (VBD commented) (SBAR (IN that) (S (BASENP (DT the) (NN blast)) (VP (VP (MD could) (RB not)) (VP (VB have) (VP (VBN been) (BASENP (NN terrorism))))))))) (PERIOD .))