Investigators for Russia special counsel Robert Muller recently sought documents from the White House on former national security adviser Michael Flynn and interviewed witnesses about "whether he was secretly paid by the Turkish government during the final months of the presidential campaign," The New York Times reported Friday.
The request, though not a formal subpoena, is "the first known instance of Mr. Mueller's team asking the White House to hand over records," the Times reports.
The article cites "people close to the investigation."
Investigators have spent "hours poring over" specifics about Flynn's dealings with a Turkish-American businessman who worked last year with Flynn and his consulting business, Flynn Intel Group.
The firm received $530,000 for a campaign to discredit an opponent of the Turkish government, an Islamic scholar now living in Pennsylvania, who was linked to a failed coup attempt last year.
Investigators are seeking to determine whether the Turkish government was involved in the payments, the Times reports.
They also are probing whether Flynn Intel kicked-backed payments to the businessman, Ekim Alptekin, "for helping conceal the source of the money," according to the report.
The efforts by Mueller's team indicate that investigation has expanded into issues involving the financial dealings of President Donald Trump's associates.
Flynn was initially under scrutiny for not registering as a foreign agent and his contacts with Russia.
In March, he retroactively registered as an agent with the Justice Department, the Times reports.
Trump fired Flynn in February over concerns about his Moscow disclosures to Vice President Mike Pence.
"For months, prosecutors have used multiple grand juries to issue subpoenas for documents related to Mr. Flynn," according to the report.
Flynn declined to comment.
Ty Cobb, special counsel to Mr. Trump, told the Times:
"We've said before we're collaborating with the special counsel on an ongoing basis.
"It's full cooperation mode as far as we are concerned."
President Trump has slammed Mueller's investigation as a "witch hunt," telling supporters in West Virginia Thursday that "the Russia story is a total fabrication."
According to the Times, Flynn's campaign to discredit the opponent of the Turkish government, Fethullah Gulen, began on Aug. 9.
Flynn Intel signed a $600,000 deal that day with Inovo BV, a Dutch company owned by Alptekin, the Turkish-American businessman.
Gulen now lives in Pennsylvania.
But the only substantive work Flynn's firm had done under the deal was an opinion piece he had written calling for better U.S.-Turkish relations and slamming Gulen as "a shady Islamic mullah," the Times reports.
Mueller's probe is focusing on two $40,000 payments that Flynn Intel made to Inovo, "said witnesses who have been interviewed in the case," according to the Times.
Investigators suspect the reimbursements were kickbacks, while Alptekin has contended that they were said that they were refunds for work that Flynn Intel had not completed.
"Ekim maintains that all payments and refunds were for unfulfilled work, and that they were legal, ethical and above board," Molly Toomey, a spokeswoman for Alptekin, told the Times.
She described the payments as "a business decision."