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First lady Melania Trump's parents, Viktor and Amalija Knavs, became U.S. citizens on Thursday, taking advantage of a program that President Trump has long railed against.

Their ceremony was private for "security reasons," attorney Michael Wildes said. Trump has decried "chain migration," where adult U.S. citizens can obtain residency for their relatives. On Nov. 1, 2017, for example, he tweeted: "CHAIN MIGRATION must end now! Some people come in, and they bring their whole family with them, who can be truly evil. NOT ACCEPTABLE!" Wildes told The New York Times "I suppose" the Knavses obtained citizenship through chain migration, but called the term a "dirtier" way of describing family-based immigration, "a bedrock of our immigration process when it comes to family reunification."

The Knavses are from Slovenia, but they now divide their time between New York City, Palm Beach, and Washington, D.C., where they stay with the Trumps in the White House. Wildes said the first lady sponsored her parents for their green cards, and once eligible, they applied for citizenship. To apply for U.S. citizenship, a person must have a green card for at least five years, plus meet the character, residency, and civic knowledge requirements. It's unclear when the Knavses obtained permanent residency in the U.S., the Times reports, but Wildes said they met the five-year requirement.

Melania Trump became a citizen in 2006, five years after she gained permanent residency by obtaining a so-called "Einstein visa," for "individuals of extraordinary ability." She began dating Donald Trump in 1998.