The First Circuit has determined that Homeland Security must prove alienage by "clear, unequivocal, and convincing” evidence, and that this standard is higher than “clear and convincing” evidence.
The full text of Rosa v. Bondi can be found here:
https://www.ca1.uscourts.gov/sites/ca1/files/opnfiles/24-1240P-01A.pdf
An amended decision can be found here:
https://www.ca1.uscourts.gov/sites/ca1/files/opnfiles/24-1240P2-01A.pdf