Special counsel Robert Mueller is investigating Michael Flynn and his son's alleged plan to forcibly remove a Muslim cleric from the United States and deliver him to Turkey, according to the Wall Street Journal.

Flynn, who is President Trump's former national security adviser, and his son Michael Flynn Jr. were allegedly were involved in a plan to deliver Fethullah Gulen to the Turkish government, which views Gulen as a political enemy and has pressed the U.S. for his extradition, The Journal reported. In exchange, Flynn and Flynn Jr. would be paid as much as $15 million.

Michael Flynn, then National Security Adviser to President Trump, attends a press conference with Japanese Prime Minister Shinzo Abe in the East Room of the White House on Feb 10, 2017.

Flynn — who was forced out of his White House job this year after revelations that he had misled officials about his conversations with the Russian ambassador — reportedly discussed the plan with Turkish government representatives last December. The meeting caught the attention of FBI, who have questioned at least four people about it.

One person who spoke to the FBI told The Journal the discussions included transporting Gulen, who lives in Pennsylvania, on a private jet to the Turkish prison island Imrali. Turkish officials, according to The Journal, first raised the possibility of removing Gulen from the country by force in an earlier meeting that took place in September. 

Extraditing Gulen is a major priority for President Recep Tayyip Erdogan. Earlier this year, Erdogan pressed Trump to send back the religious leader his government blames for an attempted coup last year and now lives in exile in Pennsylvania.

Flynn's other associations with Turkey have drawn the interest of investigators, including $530,000 in earnings from a Dutch firm with ties to the Turkish government.

More: Michael Flynn faces legal peril in Washington. In his Rhode Island hometown, he's revered.