Mass. Gen. Laws ch. 223, § 79

Personal property upon which a mortgage or lien is claimed, or which is claimed to have been sold under a contract of conditional sale reserving title in the vendor, may be attached as if unencumbered; and the mortgagee, pledgee, lienor or conditional vendor, or his assigns, may be summoned in the same action in which the property is attached as the trustee of the mortgagor, pledgor, lienee or conditional vendee, or his assigns, to answer such questions as may be put to him by the court or by its order relative to the consideration of the alleged mortgage, pledge, lien or contract of conditional sale, and the amount due thereon.