A former top White House aide was reportedly offered $15m (£11.5m) to help forcibly remove a Muslim cleric from the U.S. and deliver him to Turkey.
As part of Special Counsel Robert Mueller's probe into Russian interference with USA elections, witnesses were questioned about the alleged December 2016 meeting between Flynn and senior Turkish officials at the "21" club in NY, not far from Trump Tower, sources told NBC.
The matter is said to be under scrutiny in the wider Justice Department probe into alleged Russian election meddling.
It is unclear if the White House was aware of Flynn's discussion with Turkish government officials. And as early as September of a year ago, he had discussions with Turkish officials to find ways to deliver on the Gulen plan, according to James Woolsey, the former Central Intelligence Agency director.
Erdogan has accused Gulen of orchestrating an attempted coup against his government in 2016.
Federal Bureau of Investigation agents have asked at least four people about a meeting in mid-December where Flynn and Turkish government representatives allegedly discussed capturing Gulen, who's in exile in the US, the Journal said, citing people it didn't identify who are familiar with the FBI's inquiries. The Journal reported that the alleged discussions involved possibly moving Gulen to a Turkish prison island.
Mueller's team has also interviewed White House senior policy adviser Stephen Miller, the highest-level Trump aide known to have spoken with investigators, CNN reported on Thursday.
He also met Turkish representatives in September previous year, according to Mr Woolsey, a board member for Mr Flynn's consultancy. "But it looks as if 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, the rival for Turkey's political situation".
But a spokesman for his company has previously denied he discussed any illegal actions with the Turks.
The special counsel Robert Mueller is investigating an alleged plan that Michael Flynn, the former White House national security adviser, discussed with the Turkish government to forcibly remove a Muslim cleric from the US, The Wall Street Journal reported on Friday.
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 US ambassador.
Mueller is looking into whether the deal would have led to up to $15 million in secret payments to Flynn, sources told NBC. It was revealed afterward that during the election and transition Flynn was working as a foreign agent on behalf of Turkey. These include Flynn's partners, Bijan Kian and Brian McCauley, and Flynn's son, Michael G. Flynn.
Flynn allegedly discussed the plot at a meeting in NY in December 2016, when he was a member of then U.S. President-elect Donald Trump's transition team.