Mass. Gen. Laws ch. 269 § 6
If, by reason of the efforts made by any two or more of said magistrates or officers or by their direction to disperse such assembly, or to seize and secure the persons composing the same who have refused to disperse, though the number remaining may be less than five, any such person or any other person then present is killed or wounded, the magistrates and officers, and all persons acting by their order or under their direction, and all persons acting under the two preceding sections, shall be held guiltless, and fully justified in law; and if any of said magistrates or officers, or any person acting under or by the direction of any of the officers before mentioned, is killed or wounded, all persons so assembled, and all other persons who, when commanded or required, refused to aid and assist said magistrates or officers, shall be held answerable therefor.
Mass. Gen. Laws ch. 269, § 6