Special counsel Robert Mueller is questioning witnesses about whether former Trump administration national security adviser Michael Flynn was secretly paid by the Turkish government during the final months of the 2016 presidential campaign, according to .
The is reporting that Mueller recently asked the White House for documents related to Flynn, the first known instance of Mueller’s team asking the White House to hand over records. The newspaper said the document request was not a formal subpoena.
Flynn was President Donald Trump’s national security adviser for just 24 days, but he has become one of the central figures in the investigation of possible collusion between Russia and the Trump presidential campaign.
The reported that prosecutors have been investigating Flynn’s business dealings with a Turkish-American businessman who worked with Flynn last year, and are trying to determine if the Turkish government was behind payments to Flynn.
On Thursday, Mueller seated a grand jury, meaning his probe into Russian interference in the 2016 presidential election is growing. Investigators and prosecutors use grand juries to examine evidence, question witnesses and subpoena documents to determine if a crime has been committed.
, which first reported the grand jury news, said the panel had begun its work in recent weeks and will likely continue for months.
Trump has consistently denied any collusion between his campaign and Russia and has called Mueller’s probe “a witch hunt.”
Mueller, a former FBI director, took over the Russia probe after Trump fired his own FBI director, James Comey, in May.