(S (S (S (BASENP (PRP It)) (VP (VP (VP (VBZ is) (ADVP (RB not) (RB yet))) (ADJP (JJ clear))) (SBAR (WHNP (WP what)) (S (VP (VBD caused) (S (BASENP (DT the) (NN bus)) (VP (TO to) (VP (VP (VB swerve) (PRT (RP off))) (BASENP (DT the) (NN road)))))))))) (CONJP (COMMA ,) (CC but)) (S (SBAR (IN as) (S (BASENP (DT the) (NN crash)) (VP (VBD occurred) (PP (IN at) (NP (BASENP (NN 6:30) (NNS a.m.)) (BASENP (JJ local) (NN time))))))) (S (BASENP (DT the) (NN driver)) (VP (MD may) (VP (VB have) (VP (VP (VBN fallen) (ADJP (JJ asleep))) (PP (IN at) (BASENP (DT the) (NN wheel))))))))) (PERIOD .))