N.H. Rev. Stat. § 48:11-a
The authority of any duly authorized marshal, collector, constable, police officer or watchman of any city shall extend to any town or city in the state, provided that the chief law enforcement officer of the requesting town or city has executed with the chief law enforcement officer of the responding city a written agreement which sets forth the terms and conditions under which such assistance shall be requested or rendered. The executed agreement shall constitute authorization for every request for assistance, and for any assistance rendered in accordance with the terms and conditions of the agreement, regardless of whether the responding marshal, collector, constable, police officer or watchman is named in the agreement. In an emergency situation, the ranking on-duty law enforcement officer of a town or city is authorized to make an oral request to the ranking on-duty law enforcement officer of the responding town, subject to the terms and conditions of the written agreement, and the authority of the responding marshal, collector, constable, police officer or watchman shall extend to the requesting town or city. The written agreement shall remain in full force and effect until terminated by the mutual consent of the chief law enforcement officers of each town or city, or until 10 days after the chief law enforcement officer of one town or city has received notification from the chief law enforcement officer of the other town or city of his intention to terminate it.
RSA 48:11-a
1971, 390:1. 1981, 297:2, eff. Aug. 15, 1981.