(a) Prompt investigation required. Each claim filed against a carrier in the manner prescribed in this part shall be promptly and thoroughly investigated if investigation has not already been made prior to receipt of the claim.

(b) Supporting documents. When a necessary part of an investigation, each claim shall be supported by the bill of lading, evidence of the freight charges, if any, and either the invoice, a copy of the invoice, or an exact copy thereof or any extract made therefrom, certified by the claimant to be true and correct with respect to the property and value involved in the claim; or certification of prices or values, with trade or other discounts, allowance, or deductions, of any nature whatsoever and the terms thereof, or depreciation reflected thereon; Provided, however, That where property involved in a claim has not been invoiced to the consignee shown on the bill of lading or where an invoice does not show price or value, or where the property involved has been sold, or where the property has been transferred at bookkeeping values only, the carrier shall, before voluntarily paying a claim, require the claimant to establish the destination value in the quantity, shipped, transported, or involved; Provided, further, That when supporting documents are determined to be a necessary part of an investigation, the supporting documents are retained by the carriers for possible FMCSA inspection.

(c) Verification of loss. When an asserted claim for loss of an entire package or an entire shipment cannot be otherwise authenticated upon investigation, the carrier shall obtain from the consignee of the shipment involved a certified statement in writing that the property for which the claim is filed has not been received from any other source.