Mass. Gen. Laws ch. 246, § 14

A person summoned as trustee who admits that he has in his hands any goods, effects or credits of the defendant, or who wishes to submit the question to the court whether he is chargeable upon the facts, may make a written statement, on oath, of such facts as are material. The plaintiff may then examine him, on oath, upon written interrogatories, and the statement, interrogatories and answers shall be filed in the clerk's office.