(S (S (BASENP (DT The) (NN disaster)) (VP (VBZ is) (VP (VBN thought) (S (VP (TO to) (VP (VB be) (NP (BASENP (DT an) (NN accident)) (VP (VBN caused) (PP (IN by) (BASENP (JJ mechanical) (NN trouble))))))))))) (PERIOD .))