Say a a US citizen met foul play in a foreign country.
In general, the U.S. has no jurisdiction to prosecute crimes that occur outside the territory of the U.S. The fact that the victim is a U.S. citizen does not automatically make it an offense that the U.S. may prosecute. So what crime is this that could be prosecuted in a U.S. court?
In any event, the U.S. Code does not specify in which district the grand jury must sit. It does specify the district in which the indictment must be filed and the prosecution commenced, however. The victim's place of residence or the location of the law enforcement agency office that investigated the crime has nothing to do with it. Instead, the rule focuses on where the offender is found or resides. The applicable statute is 18 U.S.C. § 3238, which covers offenses that were not committed in any judicial district:
§3238. Offenses not committed in any district
The trial of all offenses begun or committed upon the high seas, or elsewhere out of the jurisdiction of any particular State or district, shall be in the district in which the offender, or any one of two or more joint offenders, is arrested or is first brought; but if such offender or offenders are not so arrested or brought into any district, an indictment or information may be filed in the district of the last known residence of the offender or of any one of two or more joint offenders, or if no such residence is known the indictment or information may be filed in the District of Columbia.