Mueller is investigating Mike Flynn and his son’s alleged plot to kidnap a Muslim cleric living in the U.S. and give him to Turkey for cash.

Trump’s former national security adviser and his son Junior, would have been paid up to $15 million for delivering Fethullah Gulen to the Turkish government, according to sources familiar with the investigation who spoke to the Wall Street Journal.

FBI agents have questioned at least four individuals about a meeting in December 2016, during the presidential transition, at the 21 Club in New York City, where Flynn met with representatives of the Turkish government.

That secret meeting was a follow-up to a secret Sept. 19 meeting attended by former CIA director James Woolsey, who said Flynn and others discussed “a covert step in the dead of night to whisk this guy away.”

Turkish president Recep Tayyip Erdogan views the cleric as a political enemy and has asked the U.S. to extradite him.

Mike Flynn is now facing military, congressional and criminal investigations into his financial ties to both Turkey and Russia.