Trump's ex-security aide investigated over $15m plot to kidnap Gulen for Turkey
Under the plan, Flynn, who was fired by Trump 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 US home and delivering him to the Turkish government, the Wall Street Journal reported on Friday.
Turkey suspects Gulen, who lives in self-imposed exile in Pennsylvania, of being behind the failed coup against President Recep Erdogan. He strongly denies the charges.
Mueller is examining a meeting Flynn had with senior Turkish officials weeks after Trump won the presidential race last year.
The meeting allegedly discussed a secret payout if, once in office, Flynn would engineer the deportation to Turkey of Gulen, as well as help free from prison Erdogan-linked Iranian-Turkish businessman Reza Zarrab, who is held in a Miami jail accused of helping Iran skirt US sanctions.
The discussions included details of how Gulen could be flown secretly by private jet to the isolated Turkish prison island of Imrali.
The Journal said it is not clear how far the proposal went and that there was no sign that any payments were made.
Flynn is a central figure in Mueller's Russia investigation because of conversations he had with then-Russian Ambassador Sergei Kislyak last year and because he waited until March to retroactively register with the Justice Department as a foreign agent for the work he did for a Turkish businessman.
In July, New York Times reported members of Mueller's investigation team were probing the origin of $530,000 received by Flynn's consultancy business, the Flynn Intel Group, to discover whether Turkey's government paid him to lobby against the exiled cleric.
Taking funds from a foreign government is not illegal, but failing to register it is - and any attempts to cover it up could lead to fraud charges.
Flynn, a retired lieutenant general who was previously head of the Defense Intelligence Agency, became the White House national security advisor after Trump took office on January 20, 2017.
But he was forced to resign three weeks later over his Russia contacts. Flynn has denied doing anything improper. His lawyer did not respond to requests for comment Friday.