70 ILCS 810/15
The board shall have the right and power to appoint and maintain a sufficient police force, the members of which may have and exercise police powers over the territory within such forest preserves for the preservation of the public peace, and the observance and enforcement of the ordinances and laws, such as are conferred upon and exercised by the police of organized cities and villages; but such police force, when acting within the limits of any city or village, but outside the territory owned, leased, or licensed by the district and property over which the district has easement rights, shall act in aid of the regular police force of such city or village and shall then be subject to the direction of its chief of police, city or village marshals, or other head thereof.
70 ILCS 810/15