(S (S (BASENP (NNS Scientists)) (VP (VBP say) (SBAR (S (NP (BASENP (DT the) (NN explosion)) (VP (VBN caused) (PP (IN by) (BASENP (DT the) (NN collision))))) (VP (VBD was) (ADJP (JJ massive))))))) (PERIOD .))