In an en banc decision, the Ninth Circuit confirmed that once a District Court finds that a litigant has adduced substantial credible evidence that he is a U.S. citizen, the burden shifts to the Government do demonstrate by clear and convincing evidence that the litigant is not a citizen. On appeal, an appellate court will review the District Court's factual findings regard a litigant's citizenship for clear error. In light of the contradictory evidence and credibility issues identified by the District Court, the Ninth Circuit deferred to its finding that Salvador Mondaca-Vega is not a citizen of the United States.
The full text of Mondaca-Vega v. Lynch can be found here: http://cdn.ca9.uscourts.gov/datastore/opinions/2015/12/15/03-71369.pdf