Texas Government Code - GOV'T § 418.111. Creation of the Texas Statewide Mutual Aid System

(a) The Texas Statewide Mutual Aid System is established to provide integrated statewide mutual aid response capability between local government entities without a written mutual aid agreement.

(b) A request for mutual aid assistance between local government entities is considered to be made under the system, unless the requesting and responding entities are parties to a written mutual aid agreement in effect when the request is made.

(c) This subchapter does not affect a written mutual aid agreement between local government entities in effect on or before the effective date of this subchapter or restrict the ability of local government entities to enter into a written mutual aid agreement as otherwise authorized by statute after the effective date of this subchapter.  If a request is made between local government entities that are parties to a written mutual aid agreement, the terms of that agreement control the rights and obligations of the parties.