(S (S (NP (BASENP (NNS Police)) (VP (VBG chasing) (BASENP (DT the) (NN boy)))) (VP (ADVP (RB initially)) (VP (VBD thought) (SBAR (S (BASENP (PRP they)) (VP (VBD were) (VP (VBG chasing) (BASENP (DT a) (JJ drunk) (NN driver))))))))) (PERIOD .))