The person said the questioning of Kushner earlier this month took about 90 minutes or less and was aimed in part at establishing whether Kushner had any information on Flynn that might be exculpatory. The person said multiple White House witnesses have been asked about their knowledge of Flynn, who was forced to resign from the White House in February after officials concluded he had misled them about his contacts with the Russian ambassador.
The confirmation of Kushner's interview came as prosecutors working for Mueller postponed grand jury testimony related to Flynn's private business dealings.
The reason for the postponement was not immediately clear, but it comes one week after attorneys for Flynn alerted Trump's legal team that they could no longer share information about the case. That discussion between lawyers was widely seen as a possible indication that Flynn was moving to cooperate with Mueller's investigation or attempting to negotiate a deal for himself.
An attorney for Flynn, Robert Kelner, did not immediately respond to email and phone messages Wednesday afternoon. Peter Carr, a spokesman for Mueller, declined comment.
In a statement, Kushner's attorney, Abbe Lowell, said, "Mr. Kushner has voluntarily cooperated with all relevant inquiries and will continue to do so."
The details of Kushner's questioning and the postponement of the grand jury testimony were confirmed by people familiar with Mueller's investigation. They spoke on condition of anonymity because they weren't authorized to publicly discuss the ongoing investigation.
Both developments were first reported by CNN.
The grand testimony that had been scheduled for the coming days related to Flynn's firm, Flynn Intel Group, its work with a public relations firm and interactions with congressional staff, one of the people said.
Mueller and the FBI have been interested in hearing from employees at the public relations firm, SGR LLC, because of the firm's work with Flynn Intel Group. SGR LLC, which does business as Sphere Consulting, did public relations work on a film Flynn Intel Group was working on about Turkish cleric Fethullah Gulen. The film was never completed.
Mueller was appointed by the Justice Department in May to oversee an investigation into potential coordination between Russia and the Trump campaign to influence the outcome of the 2016 presidential election. The investigation, which produced its first criminal charges last month against three former Trump campaign officials, incorporated an earlier FBI inquiry into Flynn's lobbying and investigative research work on behalf of a Turkish businessman. Flynn's firm was paid $530,000 for the work.
Sphere employees have cooperated for months with Mueller's investigation, including by turning over documents requested by investigators and sitting for voluntary interviews.
An October 2016 meeting that was expected to be the subject of the grand jury testimony has been described as a bait-and-switch carried out on behalf of Flynn's firm.
As the AP reported in March , Flynn's business partner, Bijan Kian , invited a representative of the House Homeland Security committee to Flynn Intel's offices in Alexandria, Virginia, to discuss secure communications products. But after discussing the products, the session quickly turned into a lobbying pitch that mirrored Turkish government talking points.
Kian and others involved were particularly interested in pushing for congressional hearings to investigate Gulen, whom the Turkish government has blamed for a botched coup and who has been living in exile in Pennsylvania. Gulen has denied any involvement.
Flynn Intel Group's requests for congressional hearings went nowhere.
Flynn disclosed some of the details of the meeting in a filing with the Justice Department earlier this year. According to that filing, an employee of Sphere consulting was present during the meeting.
Follow Eric Tucker and Chad Day on Twitter: https://www.twitter.com/etuckerAP and https://www.twitter.com/ChadSDay
Copyright 2018 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.