(S (S (BASENP (JJ Initial) (NNS reports)) (VP (VBP suggest) (SBAR (IN that) (S (BASENP (DT the) (NN explosion)) (VP (VBD was) (VP (VBN caused) (PP (IN by) (NP (NP (BASENP (NN methane)) (VP (VBG mixing) (PP (IN with) (BASENP (DT the) (NN air))))) (PP (IN in) (BASENP (DT the) (NN mine))))))))))) (PERIOD .))