WASHINGTON — The parents of the first lady, Melania Trump, have obtained lawful permanent residency in the United States, a lawyer for the couple confirmed Wednesday, but it remains unclear how or when the couple received their green cards.

The lack of clarity about when and how Viktor and Amalija Knavs obtained their legal residencies raises questions about whether the couple secured their residency through family-based immigration, which President Trump calls chain migration and has said he wants to restrict. Immigration experts said it would have been the most direct, and most likely, way for Mrs. Trump’s parents, formerly of Slovenia, to get their green cards.

Their immigration lawyer, Michael J. Wildes, declined to offer any details.

“It’s a privilege to help this family, but I have to respect their privacy as well,” Mr. Wildes said in a brief interview.

“Immigration is in our DNA,” he added. “We have to take great pride, no matter where somebody hails from, in that legacy.”