Viktor, right, and Amalija Knavs arrive to take the oath of citizenship with their attorney Michael Wildes in New York, Thursday, August 9.
President Trump has frequently slammed the process under which legal United States residents can sponsor the admission of family members and other people who are not close relatives.
"It stands for a bedrock of our immigration process when it comes to family reunification", he added to The Times.
The first lady's office declined to comment.
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US President Donald Trump's parents-in-law have become US citizens in a private ceremony.
President Trump has railed against family-based or "chain" immigration in the past. Politico said the parents were "expected to become more familiar figures around the White House, helping care for Barron, though they don't plan to relocate full time to Washington". "Once they had the green card, they then applied for citizenship when they were eligible", he said.
President Trump has previously spoken out against family-based migration, calling instead for a system that prioritises professionals and skilled migrants over relatives of...
It's unclear when they obtained their green cards, but usually, permanent residents have to hold green cards for five years before applying for citizenship.
She settled in NY in 1996, met the future president two years later and became his third wife in 2005.
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Melania Trump, the former Slovenian model, was able to help her parents, Viktor and Amalija Knavs, 73 and 71, obtain permanent residence likely through "chain migration" processes, Axios reports.
The Knavs' immigration attorney accompanied the couple to the ceremony.
Viktor was a auto dealer, and Amalija worked in a textile factory.
Melania grew up in a two bedroom apartment in Sevnica, a small city formerly under communist control.
The First Lady became a USA citizen in 2006, after she entered the United States in 2001 on a coveted Einstein visa for people with "extraordinary ability" when she was working as a model. She said the first lady's parents "are not part of the administration and deserve privacy".
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Together, the fires haves destroyed 75 homes and 68 other structures, while threatening more than 9,300 others. The Mendocino Complex Fire is one of over a dozen large fires burning in California at the moment.