Mass. Gen. Laws ch. 12, § 13: Districts for administration of criminal law or defense of civil actions

Section 13. For the administration of the criminal law, or for the defense of civil actions brought pursuant to chapter two hundred and fifty-eight, Suffolk county shall constitute the Suffolk district; Middlesex county, the northern district; Essex county, the eastern district; Norfolk county, the Norfolk district; Plymouth county, the Plymouth district; Bristol county, the Bristol district; Barnstable, Nantucket and Dukes counties, the Cape and Islands district; Worcester county, excluding the town of Athol, the middle district; Berkshire county, the Berkshire district; Hampden county, the Hampden district; and Franklin county, including the town of Athol, and Hampshire county, the northwestern district.