Special counsel Robert Mueller asks White House for documents on former national security adviser Michael Flynn

 Former FBI Director Robert Mueller leaves following a meeting with members of the US Senate Judiciary Committee at the US Capitol in Washington, DC, on June 21, 2017.
Former FBI Director Robert Mueller leaves following a meeting with members of the US Senate Judiciary Committee at the US Capitol in Washington, DC, on June 21, 2017. PHOTO: AFP

NEW YORK (NYTIMES) - Investigators working for the special counsel, Robert Mueller , recently asked the White House for documents related to former national security adviser Michael Flynn and have questioned witnesses about whether he was secretly paid by the Turkish government during the final months of the presidential campaign, according to people close to the investigation.

Although not a formal subpoena, the document request is the first known instance of Mueller's team asking the White House to hand over records.

In interviews with potential witnesses in recent weeks, prosecutors and FBI agents have spent hours poring over the details of Flynn's business dealings with a Turkish-American businessman who worked last year with Flynn and his consulting business, the Flynn Intel Group.

The company was paid US$530,000 (S$720,000) to run a campaign to discredit an opponent of the Turkish government who has been accused of orchestrating last year's failed coup in the country.

Investigators want to know if the Turkish government was behind those payments - and if the Flynn Intel Group made kickbacks to the businessman, Ekim Alptekin, for helping conceal the source of the money.

The line of questioning shows that Mueller's inquiry has expanded into a full-fledged examination of Flynn's financial dealings, beyond the relatively narrow question of whether he failed to register as a foreign agent or lied about his conversations and business arrangements with Russian officials.

Flynn lasted only 24 days as national security adviser, but his legal troubles now lie at the centre of a political storm that has engulfed the Trump administration.

Flynn declined to comment.

Ty Cobb, special counsel to President Donald Trump, said, "We've said before we're collaborating with the special counsel on an ongoing basis."

Investigators are examining the flow of money into and out of the Flynn Intel Group - a consulting firm Flynn founded after being forced out as the director of the Defense Intelligence Agency - according to several potential witnesses who have been interviewed by prosecutors and FBI agents.

Taking money from Turkey or any foreign government is not illegal.

But failing to register as a foreign agent is a felony, and trying to hide the source of the money by routing it through a private company or some other entity, and then paying kickbacks to the middleman, could lead to criminal charges.