Flynn was sacked after just 24 days as Trump's national security adviser over misleading Vice President Mike Pence about his conversations with Russia's ambassador to the U.S., Sergey Kislyak.
Under the plan, Mr Flynn, who was sacked by the president after just 24 days in the job, and his son, Michael Flynn Jr, were to receive up to $15 million for forcibly removing Fethullah Gulen from his Pennsylvania home and delivering him to the Turkish government.
As part of a recent investigation launched by the Special counsel Robert Mueller, Federal Bureau of Investigation (FBI) agents have asked at least four individuals about a meeting in mid-December at the "21" Club in New York City, where Mr. Flynn and representatives of the Turkish government discussed removing Mr. Gulen, according to people with knowledge of the FBI's inquiries, WSJ said.
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.
All the while new reports regarding the investigation's latest developments continue to turn up, like a recent Wall Street Journal article detailing a lucrative financial deal between the Flynns and Turkey.More news: Three WKU teams post flawless scores on Graduation Success Rate Report
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The December meeting about Mr Gulen was also reported on Friday by NBC.
Republican Rep. Devin Nunes, the chairman of the House Intelligence Committee, attended a breakfast meeting in January that Michael Flynn, the incoming national security adviser at the time, and Mevlut Cavusoglu, the Turkish foreign minister, also attended.
The sources could not confirm whether, in the meeting, Rohrabacher and Flynn discussed US policy toward Russian Federation.
Flynn is also under legal scrutiny by Mueller's team for undisclosed lobbying that he did during the presidential campaign on behalf of the Turkish government, according to sources familiar with the matter.
His lawyer later said Mr Flynn did not register because he was working for a Turkish businessman, rather than a government official.
His name has frequently come up around Mueller's investigation into whether Trump's campaign colluded with Russian Federation in its election meddling, something Trump has steadfastly denied. Flynn had written an op-ed in The Hill on Election Day calling on the US government to extradite Gulen.