I think Brian noted he probably became a US citizen through marriage. Makes the most sense to me. It does seem like a stretch that the Marines don’t know his place of birth or nationality. Even his birth date is questioned on the FBI alert. Maybe his records have received increased scrutiny with the FBI involved, and they cannot verify what Laurean had given to the Marine Corps.
Sheriff Brown said at the outset that this was going to be bizarre - much more than a killing and a burial - and I am inclined to believe him.
You don’t become a US citizen through marriage - it just makes it possible to remain in the US as a permanent resident/legal alien and perhaps makes it easier to apply/obtain citizenship by naturalization.