The Secretary may make such investigations as the Secretary deems necessary for the effective administration of this chapter or to determine whether any person subject to the provisions of this chapter has engaged or is about to engage in any acts or practices which constitute or will constitute a violation of any provisions of this chapter, or of any order, or rule or regulation issued under this chapter. For the purpose of such investigation, the Secretary is empowered to administer oaths and affirmations, subpena witnesses, compel their attendance, take evidence and require the production of any books, papers, and documents which are relevant to the inquiry. Such attendance of witnesses and the production of any such records may be required from any place in the United States. In case of contumacy by, or refusal