19 U.S. Code § 2092 - Export certification requirement
(a) Issuance by country of export
No pre-Columbian monumental or architectural sculpture or mural which is exported (whether or not such exportation is to the United States) from the country of origin after the effective date of the regulation listing such sculpture or mural pursuant to section 2091 of this title may be imported into the United States unless the government of the country of origin of such sculpture or mural issues a certificate, in a form acceptable to the Secretary, which certifies that such exportation was not in violation of the laws of that country.
(b) Procedure when certificate lacking
If the consignee of any pre-Columbian monumental or architectural sculpture or mural is unable to present to the customs officer concerned at the time of making entry of such sculpture or mural—
(1) the certificate of the government of the country of origin required under subsection (a) of this section;
(2) satisfactory evidence that such sculpture or mural was exported from the country of origin on or before the effective date of the regulation listing such sculpture or mural pursuant to section 2091 of this title; or
(3) satisfactory evidence that such sculpture or mural is not covered by the list promulgated under section 2091 of this title;
the customs officer concerned shall take the sculpture or mural into customs custody and send it to a bonded warehouse or public store to be held at the risk and expense of the consignee until such certificate or evidence is filed with such officer. If such certificate or evidence is not presented within the 90-day period after the date on which such sculpture or mural is taken into customs custody, or such longer period as may be allowed by the Secretary for good cause shown, the importation of such sculpture or mural into the United States is in violation of this chapter.