Flynn was sacked as Trump's national security adviser after 24 days over misleading Vice President Mike Pence over conversations with Sergey Kislak, Russia's USA ambassador.
Special counsel Robert Mueller is reportedly investigating an alleged plan that former White House National Security Adviser Mike Flynn discussed with the Turkish government to forcibly remove a Muslim cleric living in the United States in exchange for $15 million, according to The Wall Street Journal.
The Turkish government accuses Mr Gulen, who lives in the U.S. state of Pennsylvania, of being behind last year's failed coup.
Gulen, who has been living in self-imposed exile since 1999, strongly denies the accusations of involvement.
The discussions allegedly involved the possibility of transporting Mr. Gulen on a private jet to the Turkish prison island of Imrali, according to one of the people who has spoken to the Federal Bureau of Investigation.
Mueller is interested in the nature of the meeting between Flynn and Rohrabacher, according to NBC.
The kidnapping and return of Gulen would have netted Flynn and his son, Michael Flynn Jr.
The alleged December meeting followed one on September 19 attended by people including Berat Albayrak, who is Turkey's energy minister and the son-in-law of Erdogan, and Foreign Minister Mevlut Cavusoglu, according to the Journal.
The investigation is also looking into whether the idea of freeing Turkish-Iranian gold trader Reza Zarrab was discussed at the meeting.
Mueller's team is reportedly investigating whether kickbacks were involved in the Flynn-Inovo deal.
The discussions allegedly included how to transport Fethullah Gulen, a Muslim leader who Erdogan has accused of being behind a failed military coup to overthrow him, on a private jet to the Turkish prison island of Imrali. Friday's report describes a second meeting involving both Flynns at the 21 Club restaurant, a prohibition-era NY speakeasy patronised by Trump, in mid-December.
Flynn was a senior adviser on national security matters to the Trump campaign at the time, though he was also on a foreign government's payroll.
Participation in an effort to snatch Gulen in the US could expose Flynn to an assortment of federal charges.
It is unclear how Flynn, as national security adviser, could have successfully carried out either alleged request. The younger Flynn worked closely with his father at his lobbying firm, Flynn Intel Group. It was revealed afterward that during the election and transition Flynn was working as a foreign agent on behalf of Turkey.
The investigation is part of Mueller's expansive probe of Russian meddling in the 2016 election and possible collusion with Trump associates. Flynn had written an op-ed in The Hill on Election Day calling on the USA government to extradite Gulen.