(S (S (S (NP (BASENP (DT The) (NN cause)) (PP (IN of) (BASENP (DT the) (NN crash)))) (VP (VP (VBZ has) (RB not)) (VP (VBN been) (VP (VBN identified))))) (COMMA ,) (SBAR (IN although) (S (NP (BASENP (DT the) (NN possibility)) (PP (IN of) (NP (BASENP (DT an) (NN explosion)) (CC or) (NP (BASENP (NN terrorist) (NN attack)) (PP (IN on) (BASENP (NN board) (DT the) (NN aircraft))))))) (VP (VBZ has) (VP (VBN been) (VP (VBN ruled) (PRT (RP out)))))))) (PERIOD .))