Constitutional reforms of 1983 and 1999 of article 115, grant Mexican municipalities the right of coordination and association in order to provide public services. Inner city neighbors can obtain economic, administrative and social benefits from security services. The Metropolitan Policy of the Altamira-Tampico-Ciudad Madero Area represents a case of association of three inner municipalities in the state of Tamaulipas. In this paper we define casual models to determine the effect of the Metropolitan Police in different measures of crime. We use a time series of monthly data from September 2003 to June 2007. We find evidence indicating that, according to the official data that was used in this study, this cooperation scheme has reduced crime in Tampico.