26 C.F.R. § 20.2054-1
A deduction is allowed for losses incurred during the settlement of the estate arising from fires, storms, shipwrecks, or other casualties, or from theft, if the losses are not compensated for by insurance or otherwise. If the loss is partly compensated for, the excess of the loss over the compensation may be deducted. Losses which are not of the nature described are not deductible. In order to be deductible a loss must occur during the settlement of the estate. If a loss with respect to an asset occurs after its distribution to the distributee it may not be deducted. Notwithstanding the foregoing, no deduction is allowed under this section if the estate has waived its right to take such a deduction pursuant to the provisions of section 642(g) in order to permit its allowance for income tax purposes. See further § 1.642(g)-1 .
26 C.F.R. §20.2054-1