(S (S (S (BASENP (NNS Investigators)) (VP (VBP believe) (SBAR (S (BASENP (DT no) (JJ other) (NNS vehicles)) (VP (VBD were) (VP (VBN involved))))))) (CONJP (COMMA ,) (CC but)) (S (BASENP (DT the) (JJ exact) (NNS circumstances)) (VP (VBP remain) (ADJP (JJ unclear))))) (PERIOD .))