Mass. Gen. Laws ch. 276 § 52
The commissioner of correction shall from time to time make such rules relative to the diet, size of cells, amount of liberty and exercise, correspondence, visits and such other matters as he considers necessary regulating the treatment of witnesses held in jail as will secure their clear distinction and separation from other prisoners so far as possible, consistent with their safe custody and the prevention of tampering with their testimony. Said commissioner may, with the approval of the district attorney, remove such witnesses from the jail where they are confined to a jail in another county, and shall, at the request of the district attorney, cause them to be returned to the jail whence they were removed. The proceedings for such removal shall be the same as for the removal of prisoners from one jail or house of correction to another. The cost of support of a witness so removed and of both removals shall be paid by the county whence he is removed.
Mass. Gen. Laws ch. 276, § 52