For the purposes of this chapter, the Secretary shall at all reasonable times have access to, for the purpose of examination, and the right to copy any documentary evidence of any person with respect to whom such authority is exercised; and the Secretary shall have power to require by subpena the attendance and testimony of witnesses and the production of all such documentary evidence relating to any matter under investigation by the Secretary, and may administer oaths and affirmations, examine witnesses, and receive evidence.