WASHINGTON—Special counsel Robert Mueller is investigating former White House national security adviser Mike Flynn’s work on an unfinished film financed by Turkish interests as part of its wider probe into whether Mr. Flynn improperly concealed financial ties to Turkey and to Russia, according to people familiar with the matter.

The focus on the film comes amid signs that Mr. Flynn may be working on a deal with Mr. Mueller. Mr. Flynn’s lawyers this week stopped cooperating with White House attorneys defending the president, according to other people familiar with the matter, a possible sign that Mr. Flynn is sharing information with investigators.

The Federal Bureau of Investigation is preparing to interview consultants hired by Mr. Flynn to work on a documentary film targeting an exiled Turkish cleric who Ankara accuses of trying to overthrow the country’s president, according to people familiar with the investigation. Details of the unfinished film project were first reported by The Wall Street Journal in May.

The film was at the heart of Mr. Flynn’s work for Turkish interests while he served as top adviser on then-candidate Donald Trump’s presidential campaign. While working to get Mr. Trump elected, Mr. Flynn’s company signed a $530,000 contract with a Turkish businessman with close ties to President Recep Tayyip Erdogan.

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Mr. Flynn and his business partner, Bijan Kian, hired consultants to create a film attacking the Muslim cleric, Fethullah Gulen, who has been accused of orchestrating a failed July 2016 coup attempt in Turkey from his home in rural Pennsylvania. Mr. Gulen has denied playing any role in the short-lived putsch.


Mr. Flynn’s consulting firm, the Flynn Intel Group, tried to conceal its role in the documentary, said David Enders, a Beirut-based freelance journalist hired to film interviews. Mr. Enders said that Mr. Kian told him that he didn’t want anyone to know who was behind the movie.

“He said: ‘We don’t want anyone to know the Flynn Intel Group has anything to do with this,’ ” Mr. Enders told the Journal in May.

Mr. Kian didn’t respond to a request for comment. Peter Carr, a spokesman for Mr. Mueller, likewise declined to comment. Mr. Flynn’s lawyer, Robert Kelner, wasn’t immediately available for comment on the matter.

FBI agents have contacted Mr. Enders and Rudi Bakhtiar, a former CNN anchor hired to work on the documentary, to ask them about their roles in the venture, according to people familiar with the matter.


Others who have already spoken to the FBI have said that investigators have been asking detailed questions about Mr. Kian, former vice chair of the now-defunct Flynn Intel Group.

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Mr. Flynn waited until March of this year to file federal documents showing that he was paid to represent Turkish interests. He is now the focus of investigators looking into whether he improperly concealed his financial ties to Turkey as well as Russia.

Mr. Mueller also is probing the alleged role played by Mr. Flynn and his son, Michael Flynn Jr., in a plan to forcibly remove Mr. Gulen and deliver him to Turkey in return for millions of dollars, the Journal previously reported. Under the alleged proposal, Mr. Flynn and his son were to be paid as much as $15 million for delivering Mr. Gulen to the Turkish government.

Mr. Flynn’s lawyer and the Turkish government have said the plot allegations are false. A lawyer for Mr. Flynn Jr. declined to comment.

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The developments come as lawyers for Mr. Flynn abruptly stopped cooperating with White House attorneys defending the president, according to two people familiar with the matter, raising the possibility that the former national security adviser could be looking to cut a deal.


Mr. Flynn’s lawyer, Mr. Kelner, declined to comment on the decision to no longer discuss the special counsel’s investigation of Trump associates’ ties to Russiawith the president’s legal team that they could, according to two people familiar with the matter. the move.

Mr. Flynn’s lawyers earlier had entered into a joint defense agreement with the president’s legal team that allowed them to share information surrounding Mr. Mueller’s investigation that would otherwise be subject to attorney-client privilege. On Wednesday, Mr. Flynn’s lawyers called John Dowd, who heads Mr. Trump’s legal team, and told him they were ending the agreement, according to the people.

There are several possible reasons for taking this step, which was first reported by the New York Times on Thursday.

Terminating a joint defense agreement could mean that Mr. Flynn, a longtime campaign aide who was ousted from the Trump administration in February, is cooperating in the special counsel’s investigation. It would be an ethical breach for one party of a joint defense agreement to begin cooperating with an investigation while continuing to share information.


Mr. Flynn’s move to end the agreement could also mean that he has entered plea deal discussions with Mr. Mueller’s team, which would mean that Mr. Mueller is moving toward charging the former national security adviser. The special counsel has already charged two former Trump campaign aides—Paul Manafort and Rick Gates, who both pleaded not guilty—and has obtained a guilty plea from a third, George Papadopoulos.

“It was not unexpected to us that Gen. Flynn’s lawyers might be entering into discussions with the special counsel [about] potential pleas,” said Jay Sekulow, a member of the White House team. “No one should draw the conclusion from that that this means anything about Gen. Flynn cooperating against the president.”

Mr. Mueller’s team is investigating whether Trump associates colluded in Russia’s efforts to interfere in the 2016 U.S. election, and has a broad mandate to examine any matters that arise from that probe. Mr. Trump has denied any collusion by him or his campaign, and Moscow has denied meddling in the election.

—Del Quentin Wilber and James Grimaldi contributed to this article.

Write to Dion Nissenbaum at dion.nissenbaum@wsj.com and Rebecca Ballhaus at Rebecca.Ballhaus@wsj.com