If, at the time of arrival in the United States, the residence or the legal domicile of an eligible person appearing to be in need of care and treatment in a mental hospital is known to be in a State, or whenever thereafter such a person’s residence or legal domicile in a State is ascertained, the Secretary shall, if the person is then under his care (whether directly or pursuant to a contract or other arrangement under section 322 or 324 of this title), endeavor to arrange with the proper authorities of such State, or of a political subdivision thereof, for the assumption of responsibility for the care and treatment of such person by such authorities and shall, upon the making of such arrangement in writing, transfer and release such person to such authorities. In the event the State of the residence or legal domicile of an eligible person cannot be ascertained, or the Secretary is unable to arrange with the proper authorities of such State, or of a political subdivision thereof, for the assumption of responsibility for his care and treatment, the Secretary may, if he determines that the best interests of such person will be served thereby, transfer and release the eligible person to a relative who agrees in writing to assume responsibility for such person after having been fully informed as to his condition.