US citizenship is the status of a natural person as a legal member of the United States. It implies rights, privileges, immunities and economic benefits including federal assistance. According to the Citizenship Clause of the Fourteenth Amendment to the Constitution, a person is an United States citizen automatically at birth in the United States, by being born in another country as a child of American parents or grandparents or having been born in another country through a process of nationalization.
Therefore, Elsa could have been born in the United States, being the daughter of foreigners; or have nationalized before their parents.