21 U.S.C. § 851 : US Code - Section 851: Proceedings to establish prior convictions

    (a) Information filed by United States Attorney
      (1) No person who stands convicted of an offense under this part
    shall be sentenced to increased punishment by reason of one or more
    prior convictions, unless before trial, or before entry of a plea
    of guilty, the United States attorney files an information with the
    court (and serves a copy of such information on the person or
    counsel for the person) stating in writing the previous convictions
    to be relied upon. Upon a showing by the United States attorney
    that facts regarding prior convictions could not with due diligence
    be obtained prior to trial or before entry of a plea of guilty, the
    court may postpone the trial or the taking of the plea of guilty
    for a reasonable period for the purpose of obtaining such facts.
    Clerical mistakes in the information may be amended at any time
    prior to the pronouncement of sentence.
      (2) An information may not be filed under this section if the
    increased punishment which may be imposed is imprisonment for a
    term in excess of three years unless the person either waived or
    was afforded prosecution by indictment for the offense for which
    such increased punishment may be imposed.
    (b) Affirmation or denial of previous conviction
      If the United States attorney files an information under this
    section, the court shall after conviction but before pronouncement
    of sentence inquire of the person with respect to whom the
    information was filed whether he affirms or denies that he has been
    previously convicted as alleged in the information, and shall
    inform him that any challenge to a prior conviction which is not
    made before sentence is imposed may not thereafter be raised to
    attack the sentence.
    (c) Denial; written response; hearing
      (1) If the person denies any allegation of the information of
    prior conviction, or claims that any conviction alleged is invalid,
    he shall file a written response to the information. A copy of the
    response shall be served upon the United States attorney. The court
    shall hold a hearing to determine any issues raised by the response
    which would except the person from increased punishment. The
    failure of the United States attorney to include in the information
    the complete criminal record of the person or any facts in addition
    to the convictions to be relied upon shall not constitute grounds
    for invalidating the notice given in the information required by
    subsection (a)(1) of this section. The hearing shall be before the
    court without a jury and either party may introduce evidence.
    Except as otherwise provided in paragraph (2) of this subsection,
    the United States attorney shall have the burden of proof beyond a
    reasonable doubt on any issue of fact. At the request of either
    party, the court shall enter findings of fact and conclusions of
    law.
      (2) A person claiming that a conviction alleged in the
    information was obtained in violation of the Constitution of the
    United States shall set forth his claim, and the factual basis
    therefor, with particularity in his response to the information.
    The person shall have the burden of proof by a preponderance of the
    evidence on any issue of fact raised by the response. Any challenge
    to a prior conviction, not raised by response to the information
    before an increased sentence is imposed in reliance thereon, shall
    be waived unless good cause be shown for failure to make a timely
    challenge.
    (d) Imposition of sentence
      (1) If the person files no response to the information, or if the
    court determines, after hearing, that the person is subject to
    increased punishment by reason of prior convictions, the court
    shall proceed to impose sentence upon him as provided by this part.
      (2) If the court determines that the person has not been
    convicted as alleged in the information, that a conviction alleged
    in the information is invalid, or that the person is otherwise not
    subject to an increased sentence as a matter of law, the court
    shall, at the request of the United States attorney, postpone
    sentence to allow an appeal from that determination. If no such
    request is made, the court shall impose sentence as provided by
    this part. The person may appeal from an order postponing sentence
    as if sentence had been pronounced and a final judgment of
    conviction entered.
    (e) Statute of limitations
      No person who stands convicted of an offense under this part may
    challenge the validity of any prior conviction alleged under this
    section which occurred more than five years before the date of the
    information alleging such prior conviction.