President Trump’s former national security adviser, Michael Flynn, and his son were reportedly offered $15m to help forcibly remove a Muslim cleric from the US and deliver him to Turkey.
Father and son discussed the alleged plot with Turkish representatives, NBC News and Wall Street Journal report.
The matter is said to be under scrutiny in the wider Justice Department probe into alleged Russian election meddling.
Mr Flynn quit after misleading the White House about meeting an envoy.
The alleged plot to remove the cleric, Fethullah Gulen, was first revealed in March 2017 by former CIA director James Woolsey.
The Turkish government accuses Mr Gulen, who lives in the US state of Pennsylvania, of being behind last year’s failed coup.
He is viewed as chief political rival to Turkish President Recep Tayyip Erdogan, who has repeatedly called for Mr Gulen’s extradition from the US.
According to the Wall Street Journal, special counsel Robert Mueller’s investigation is focusing on a meeting in mid-December between Mr Flynn and Turkish officials in New York.
Mr Flynn reportedly discussed having Mr Gulen transported on a private jet to the Turkish prison island of Imrali.
Mr Flynn was serving on the White House transition team during the reported meeting, which came a month before he joined the Trump administration.
He also met Turkish representatives in September last year, according to Mr Woolsey, a board member for Mr Flynn’s consultancy.
Mr Woolsey has previously told CNN that in September, “there was at least some strong suggestion by one or more of the Americans present at the meeting that we would be able, the United States would be able, through them, to be able to get hold of Gulen”.
NBC reported that federal investigators are also looking into whether Mr Flynn tried to push for the return of Mr Gulen to Turkey during his time as White House national security adviser.