(S (S (BASENP (NNS Police)) (VP (VBD had) (VP (VP (VP (VBN warned) (BASENP (PRP them))) (PP (IN about) (S (VP (VBG crossing) (PP (IN into) (BASENP (DT the) (JJ disputed) (NN land))))))) (ADVP (RB before))))) (PERIOD .))