(S (S (S (NP (BASENP (DT The) (NN cause)) (PP (IN of) (BASENP (NN death)))) (VP (VBZ has) (VP (VBN been) (VP (VBN reported) (S (VP (TO to) (VP (VB be) (BASENP (DT an) (NN explosion))))))))) (CONJP (COMMA ,) (CC but)) (S (BASENP (DT the) (NN source)) (VP (VBZ is) (ADJP (JJ unclear))))) (PERIOD .))