Washington: Investigators working for the special counsel, Robert Mueller, recently asked the White House for documents related to former national security adviser Michael Flynn.

They have also 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.