Mass. Gen. Laws ch. 41 § 99J
Each regional police district commission shall organize a regional district police department, and such department shall begin operations and otherwise take effect on January first following acceptance of the provisions of the regional police district law. Each regional district police department so established shall succeed any existing town police departments, whether fully organized or not, within its district, and each such town police department shall cease to exist on December thirty-first following the acceptance of the provisions of the regional police district law. Any pending criminal prosecutions of the police of the member towns shall be assumed by the regional district police department, and all town police employees who immediately prior to the effective date of the district held positions classified under chapter thirty-one or had tenure in their positions by special act shall be transferred to the service of the district police department upon the establishment of said department; every such transfer to be without impairment of civil service status, seniority, retirement and other rights of the employee, without interruption of his service within the meaning of said chapter thirty-one or special act, and without reduction in his compensation and salary grade, notwithstanding any change in his title or duties made as a result of such transfer. Said district police department shall be considered a single town police department as that phrase is used or implied in the general laws, and all general laws applicable to town police departments shall apply to such district police departments. If a majority of towns in said district shall vote to accept the provisions of chapter thirty-one, at annual elections held therein, with respect to such district, by the method provided in section fifty-four of said chapter, the provisions of said chapter shall apply to said district. Any such district department shall be under the supervision of an officer known as the chief of police.
The chief and other police officers of all regional police districts shall have, throughout their respective districts, unless a different meaning is clearly apparent from the language or context, all the powers, duties, immunities and privileges conferred by the General Laws on chiefs and police officers of the towns.
Mass. Gen. Laws ch. 41, § 99J