26 U.S.C. § 7602 : US Code - Section 7602: Examination of books and witnesses

    (a) Authority to summon, etc.
      For the purpose of ascertaining the correctness of any return,
    making a return where none has been made, determining the liability
    of any person for any internal revenue tax or the liability at law
    or in equity of any transferee or fiduciary of any person in
    respect of any internal revenue tax, or collecting any such
    liability, the Secretary is authorized - 
        (1) To examine any books, papers, records, or other data which
      may be relevant or material to such inquiry;
        (2) To summon the person liable for tax or required to perform
      the act, or any officer or employee of such person, or any person
      having possession, custody, or care of books of account
      containing entries relating to the business of the person liable
      for tax or required to perform the act, or any other person the
      Secretary may deem proper, to appear before the Secretary at a
      time and place named in the summons and to produce such books,
      papers, records, or other data, and to give such testimony, under
      oath, as may be relevant or material to such inquiry; and
        (3) To take such testimony of the person concerned, under oath,
      as may be relevant or material to such inquiry.
    (b) Purpose may include inquiry into offense
      The purposes for which the Secretary may take any action
    described in paragraph (1), (2), or (3) of subsection (a) include
    the purpose of inquiring into any offense connected with the
    administration or enforcement of the internal revenue laws.
    (c) Notice of contact of third parties
      (1) General notice
        An officer or employee of the Internal Revenue Service may not
      contact any person other than the taxpayer with respect to the
      determination or collection of the tax liability of such taxpayer
      without providing reasonable notice in advance to the taxpayer
      that contacts with persons other than the taxpayer may be made.
      (2) Notice of specific contacts
        The Secretary shall periodically provide to a taxpayer a record
      of persons contacted during such period by the Secretary with
      respect to the determination or collection of the tax liability
      of such taxpayer. Such record shall also be provided upon request
      of the taxpayer.
      (3) Exceptions
        This subsection shall not apply - 
          (A) to any contact which the taxpayer has authorized;
          (B) if the Secretary determines for good cause shown that
        such notice would jeopardize collection of any tax or such
        notice may involve reprisal against any person; or
          (C) with respect to any pending criminal investigation.
    (d) No administrative summons when there is Justice Department
      referral
      (1) Limitation of authority
        No summons may be issued under this title, and the Secretary
      may not begin any action under section 7604 to enforce any
      summons, with respect to any person if a Justice Department
      referral is in effect with respect to such person.
      (2) Justice Department referral in effect
        For purposes of this subsection - 
        (A) In general
          A Justice Department referral is in effect with respect to
        any person if - 
            (i) the Secretary has recommended to the Attorney General a
          grand jury investigation of, or the criminal prosecution of,
          such person for any offense connected with the administration
          or enforcement of the internal revenue laws, or
            (ii) any request is made under section 6103(h)(3)(B) for
          the disclosure of any return or return information (within
          the meaning of section 6103(b)) relating to such person.
        (B) Termination
          A Justice Department referral shall cease to be in effect
        with respect to a person when - 
            (i) the Attorney General notifies the Secretary, in
          writing, that - 
              (I) he will not prosecute such person for any offense
            connected with the administration or enforcement of the
            internal revenue laws,
              (II) he will not authorize a grand jury investigation of
            such person with respect to such an offense, or
              (III) he will discontinue such a grand jury
            investigation,

            (ii) a final disposition has been made of any criminal
          proceeding pertaining to the enforcement of the internal
          revenue laws which was instituted by the Attorney General
          against such person, or
            (iii) the Attorney General notifies the Secretary, in
          writing, that he will not prosecute such person for any
          offense connected with the administration or enforcement of
          the internal revenue laws relating to the request described
          in subparagraph (A)(ii).
      (3) Taxable years, etc., treated separately
        For purposes of this subsection, each taxable period (or, if
      there is no taxable period, each taxable event) and each tax
      imposed by a separate chapter of this title shall be treated
      separately.
    (e) Limitation on examination on unreported income
      The Secretary shall not use financial status or economic reality
    examination techniques to determine the existence of unreported
    income of any taxpayer unless the Secretary has a reasonable
    indication that there is a likelihood of such unreported income.