26 C.F.R. § 20.6018-2

It is required that the duly qualified executor or administrator shall file the return. If there is more than one executor or administrator, the return must be made jointly by all. If there is no executor or administrator appointed, qualified and acting within the United States, every person in actual or constructive possession of any property of the decedent situated in the United States is constituted an executor for purposes of the tax (see § 20.2203-1), and is required to make and file a return. If in any case the executor is unable to make a complete return as to any part of the gross estate, he is required to give all the information he has as to such property, including a full description, and the name of every person holding a legal or beneficial interest in the property. If the executor is unable to make a return as to any property, every person holding a legal or beneficial interest therein shall, upon notice from the district director, make a return as to that part of the gross estate. For delinquency penalty for failure to file return, see section 6651 and § 301.6651-1 of this chapter (Regulations on Procedure and Administration). For criminal penalties for failure to file a return and filing a false or fraudulent return, see sections 7203, 7206, 7207, and 7269.