The Trump administration has increasingly questioned the citizenship of Americans with United States birth certificates, revoking or denying passports to “hundreds, and possibly thousands” of people and placing some in deportation proceedings, the Washington Post reported Wednesday.

The surge in citizenship challenges has become a theme in the Trump administration. U.S. Citizenship and Immigration Services launched a highly public denaturalization task force earlier this year to investigate naturalization-related fraud and refer cases to the Justice Department.

But the process described by the Post is altogether different and concerns instances in decades past of fraudulent U.S. birth certificates being issued for babies delivered by midwives on Mexican soil, near the U.S.-Mexico border. Following a settlement with the ACLU and other civil rights groups in 2009, passport denials based on suspicious birth certificates had “mostly” ended, the Post reported, or were otherwise quickly resolved.

Under Trump, according to the report, “passport denials and revocations appear to be surging.” The associated legal costs are steep, and the bureaucracy daunting. The Post described the case of “Juan,” a Brownsville-born former Army private and Border Patrol cadet, now a prison guard. His application to renew his passport was denied, and the government said it doubted his citizenship:

When Juan, the former soldier, received a letter from the State Department telling him it wasn’t convinced that he was a U.S. citizen, it requested a range of obscure documents — evidence of his mother’s prenatal care, his baptismal certificate, rental agreements from when he was a baby. He managed to find some of those documents but weeks later received another denial. In a letter, the government said the information “did not establish your birth in the United States.”

The State Department denied in a statement to the Post that it had “changed policy or practice regarding the adjudication of passport applications,” and said “the U.S.-Mexico border region happens to be an area of the country where there has been a significant incidence of citizenship fraud.”

Passport applicants “who have birth certificates filed by a midwife or other birth attendant suspected of having engaged in fraudulent activities, as well as applicants who have both a U.S. and foreign birth certificate, are asked to provide additional documentation establishing they were born in the United States,” the State Department told the Post.

The government didn’t say how many midwives it considered suspicious, the Post said, nor how many passports it has denied.

Read the Post’s full report here.