Special counsel Robert Mueller's team has asked the White House for documents concerning former national security adviser Mike Flynn, the first known instance of the investigation doing so, according to a report Friday.

In particular, the probe appears to be interested in Flynn's ties to the Turkish government.

The New York Times reported that the Mueller team, which is examining Russian interference in the 2016 election and possible collusion with the Trump campaign, has questioned witnesses about whether Flynn was paid by Turkey in the final months of the presidential campaign.

The news comes amid new revelations this week that Mueller has impaneled multiple grand juries in relation to his Russia probe, indicating that his efforts are ramping up.

Investigators want to know whether Flynn was paid by the government through a Turkish-American businessman, Ekim Alptekin. Flynn and his consulting business, the Flynn Intel Group, worked with Alptekin last year. A payment of $530,000 was put towards damaging the reputation of Fethullah Gulen, a cleric residing in the U.S., who Turkish President Recep Tayyip Erdogan blames for being behind and attempting coup last year.

Witnesses being interviewed say that Mueller's investigators are interested to two potential kickback payments of $40,000 each from Flynn Intel Grou to Inovo BV, a Dutch company owned by Alptekin.

Flynn has run into trouble filling fully revealing financial-disclosure forms, this week filing his third one. The newer versions show money he received for Russia-related groups and his contract work with a data mining company that helped Trump's campaign.

Flynn served a few weeks as Trump's national security adviser until he was ousted after it was revealed he misled the administration about his communications with a Russian ambassador Sergey Kislyak. His contact with Russian officials has already come under scrutiny by investigators looking into Russian interference.

A federal grand jury in Alexandria, Va., looking into Flynn and his foreign lobbying was taken over by Mueller's team, Reuters reported in June.

While Flynn declined to comment to the Times, Ty Cobb, special counsel to Trump, said in a brief statement, "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," he added.