Mass. Gen. Laws ch. 276 § 10A
Any member of a duly organized state, county or municipal peace unit of another state of the United States the laws of which contain provisions substantially equivalent to the provisions of this and the following section, who enters this commonwealth in fresh pursuit, and continues herein in such fresh pursuit, of a person in order to arrest him on the ground that he has committed a felony in such other state shall have the same authority to arrest and hold in custody such person as members of a duly organized state, county or municipal peace unit of this commonwealth have to arrest and hold in custody a person on the ground that he has committed a felony in this commonwealth. This section shall not be construed so as to make unlawful any arrest in this commonwealth which would otherwise be lawful.
Mass. Gen. Laws ch. 276, § 10A