Reuters
Mr Flynn denies discussing anything illegal
A former top White House aide was reportedly offered $ 15m (£ 11.5m) to help forcibly remove a Muslim cleric from the US and deliver him to Turkey.
Ex-national security adviser Michael Flynn and his son discussed the alleged plot with Turkish representatives, NBC News and Wall Street Journal report.
The Justice Department probe into alleged Russian election meddling.
Mr Flynn quit after misleading the White House about meeting an envoy.
The former plot to remove the cleric, Fethullah Gulen, was first revealed in March 2017 by former CIA director James Woolsey
The Turkish government accuses Mr Gulen, who lives in the US state of Pennsylvania, of being behind last year's failed coup.
He is viewed as chief political rival to Turkish President Recep Tayyip Erdogan, who has repeat edited for Mr. Gulen's extradition from the US
According to the Wall Street Journal, special counsel Robert Mueller's investigation is focusing on a meeting in mid-December between Mr Flynn and Turkish officials in New York.
Fethullah Gulen: Powerful but reclusive Turkish cleric
Mr. Flynn was serving on the White House transition team during the reported meeting, which came a month before he joined the White House
He also met Turkish representative in September last year, according to Mr. Woolsey, a board member for Mr Flynn's consultancy.
Mr. Woolsey has previously told CNN that in September, "there was at least some strong sugg "
NBC reported that the federal investigators are also looking into whether Mr Flynn tried to push for the return of Mr Gulen to Turkey during his time as White House national security adviser.
Mr Flynn's attorney did not immediately respond to a request for comment from the BBC.
But a spokesman for his company has previously denied he discussed any illegal actions with the Turks.
Barry Coburn, a lawyer for Mr Flynn's son,
Mr. Flynn Snr was the first aide in Mr. Trump's White House to resign, after only 23 days on the job.
The retired lieutenant general had admitted lying to Vice President Mike Pence about a meeting with the Russian ambassador in which the lifting of US sanctions was discussed.
Mr Flynn also failed to register as a lobbyist for the Turkish government while he was seeking White House security clearance.
In 2016, his consultancy Flynn Intel Group was paid $ 530,000 for lobbying on behalf of the Turkish government - work which required him to register as "a foreign agent".
His lawyer later said Mr Flynn did not register because he was working for a Turkish businessman, rather than a government official.
Both US media outlets also reported that Mr. Flynn and the meeting attendees, Michel Flynn Jr., who worked closely with him at Flynn Intel Group.
discussed a way to free Turkish-Iranian gold trader Reza Zarrab, who is in a US jail over charges that he evaded US sanctions on Iran.
