Special counsel Robert Mueller is investigating the role played by former national security adviser Michael Flynn in an alleged plot to forcibly remove a Muslim cleric living legally in the U.S. to Turkey in exchange for millions of dollars, the Wall Street Journal reported Friday.

The alleged plot described by the Journal involves a direct quid quo, in which Flynn and his son, Michael Flynn, Jr., would receive up to $15 million for successfully delivering Fetullah Gulen to the Turkish government.

At least four people have been interviewed by the FBI about a December meeting at New York’s 21 Club in which Flynn, who had already been named as Donald Trump’s national security adviser, discussed the plan with representatives of Turkey’s government, according to the report.

NBC News, which confirmed the story, noted that multiple federal charges could be brought if a U.S. government official agreed to be bribed to secretly carry out the bidding of a foreign government. NBC’s report notes that the plan would have apparently been carried out after Flynn was installed in the White House.

The special counsel and Flynn Jr.’s lawyer, Barry Coburn, declined the Journal’s request for comment. Flynn’s attorney Robert Kelner did not respond.

Mueller is already investigating Flynn and his now-defunct consulting firm, Flynn Intel Group, for a host of matters related to their work for foreign governments. The retired lieutenant general retroactively registered as a foreign agent for a separate project he carried out on behalf of Turkey while serving on the Trump campaign. His consulting firm received some $530,000 from a Turkish businessman to produce negative PR materials on Gulen.

As the Journal noted, he had also held an earlier meeting with Turkish representatives on Sept. 19, 2016 about forcibly removing the exiled cleric. Former CIA Director James Woolsey previously told the newspaper that he attended that initial meeting at a New York hotel and was concerned to hear about what sounded like an illegal plot to “whisk this guy away.”

Reuters recently reported that Woolsey, who was then a member of Flynn’s firm and an adviser to the Trump campaign, then held his own meeting with Turkish businessmen on Sept. 20 in which he offered to help discredit Gulen in exchange for $10 million.

The special counsel’s team has interviewed Woolsey about the Sept. 19 meeting, according to his spokesman. Two people familiar with the probe told NBC that a number of other witnesses “with knowledge of Flynn’s business activities” were also coming in for interviews over the next week.

TPM attempted to reach a number of Flynn’s business associates this week to ask about their contacts with the special counsel, but received few responses.

Reached by telephone Tuesday, Flynn Intel Group’s former general counsel, Bob Kelley, said he had not been called in for an interview.

Asked if he was surprised by the recent indictment of former Trump campaign officials Paul Manafort and Rick Gates on financial crimes charges, Kelley took a long pause, then hung up the phone.

Editor’s note: The original version of this story incorrectly stated that Bob Kelley, rather than former campaign official Rick Gates, had been indicted alongside Manafort.