Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 40, § 8G

A city or town which accepts this section may enter into an agreement with another city or town, or other cities and towns including cities and towns in states contiguous to the commonwealth, to provide mutual aid programs for police departments to increase the capability of such departments to protect the lives, safety, and property of the people in the area designated in the agreement. Said agreement may include the furnishing of personal services, supplies, materials, contractual services, and equipment when the resources normally available to any municipality in the agreement are not sufficient to cope with a situation which requires police action. When providing such mutual aid, police officers shall have all the immunities and powers granted to them in the municipalities that employ them, including, but not limited to, powers of arrest.

While in transit to, returning from, and during a mutual aid response for another city or town, a participating police officer shall maintain the right of indemnification granted by law, or by his home city or town, or both, for all claims arising out of any action within the scope of his employment in accordance with a valid mutual aid agreement. A mutual aid agreement may provide for methods of activation or requesting and responding to mutual aid requests. The agreement may also address issues such as pay and benefits for officers, insurance, indemnification, injury compensation and other operational matters related to police services provided for a mutual aid response. Nothing in this paragraph shall be construed to expand or otherwise modify existing police powers as provided by law or any existing mutual aid agreement or both.