19 C.F.R. § 158.3
Allowance shall be made in the assessment of duties for lost or missing packages of merchandise included in an entry summary whenever it is established to the satisfaction of the Center director before the liquidation of the entry summary becomes final that the merchandise claimed to be lost or missing was not "permitted." A claim for such allowance shall be made on Customs Form 5931, in triplicate, executed by the importer and the importing carrier or bonded carrier, as appropriate. When the importing or bonded carrier refuses to execute the Customs Form 5931, a claim may be allowed if the importer properly executes the Customs Form 5931 and attaches copies of the dock receipt or other document evidencing nonreceipt of the lost or missing packages.
19 C.F.R. §158.3