26 U.S.C. § 6018

(a) Returns by executor (1) Citizens or residents In all cases where the gross estate at the death of a citizen or resident exceeds the basic exclusion amount in effect under section 2010(c) for the calendar year which includes the date of death, the executor shall make a return with respect to the estate tax imposed by subtitle B. (2) Nonresidents not citizens of the United States In the case of the estate of every nonresident not a citizen of the United States if that part of the gross estate which is situated in the United States exceeds $60,000, the executor shall make a return with respect to the estate tax imposed by subtitle B. (3) Adjustment for certain gifts The amount applicable under paragraph (1) and the amount set forth in paragraph (2) shall each be reduced (but not below zero) by the sum of— (A) the amount of the adjusted taxable gifts (within the meaning of section 2001(b)) made by the decedent after December 31, 1976 , plus (B) the aggregate amount allowed as a specific exemption under section 2521 (as in effect before its repeal by the Tax Reform Act of 1976) with respect to gifts made by the decedent after September 8, 1976 . (b) Returns by beneficiaries If the executor is unable to make a complete return as to any part of the gross estate of the decedent, he shall include in his return a description of such part and the name of every person holding a legal or beneficial interest therein. Upon notice from the Secretary such person shall in like manner make a return as to such part of the gross estate.