WASHINGTON — President Trump’s son-in-law and adviser, Jared Kushner, met this month with investigators working for Robert S. Mueller III, the special counsel, and answered questions about a meeting with a Russian ambassador during the presidential transition, according to a person briefed on the investigation.

The questions focused on a meeting in December between Mr. Kushner, the ambassador and Michael T. Flynn, who at the time was the president’s incoming national security adviser, the person said on Wednesday.

Prosecutors also asked Mr. Kushner about other interactions between Mr. Flynn and the Russian government, the person briefed on the investigation said. Mr. Flynn was fired in February after misleading Vice President Mike Pence about his conversations during the transition with Sergey I. Kislyak, who was then the Russian ambassador to the United States.

Reached for comment on Wednesday night, Mr. Kushner’s lawyer, Abbe D. Lowell, said, “Mr. Kushner has voluntarily cooperated with all relevant inquiries and will continue to do so.” The special counsel’s interview of Mr. Kushner was first reported by CNN.