Mo. Rev. Stat. § 144.645
For the purpose of ascertaining the correctness of any return, or determining the amount of tax due from any taxpayer, the director of revenue or any employee designated in writing by the director of revenue may hold investigations and hearings concerning any matters covered by this law and may examine any books, papers, records or memoranda of any taxpayer bearing upon the amount of business done and may require of any person the production of books, papers and records bearing upon the tax levied by this law or the attendance of any person for the purpose of taking his testimony with respect to any matter within the purview of the director of revenue. In the conduct of any investigation or hearing, neither the director of revenue nor any employee thereof is bound by the technical rules of evidence and no informality in any proceeding, or in the manner of taking testimony, shall invalidate any order, decision, rule or regulation made or confirmed by the director of revenue. The director of revenue or any person designated by him may administer oaths in all matters relating to the administration of this law.
§ 144.645, RSMo