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Attorneys for former White House National Security Adviser Michael Flynn have informed President Donald Trump's legal team that they can no longer discuss Special Counsel Robert Mueller's investigation into Russian activities during the 2016 election campaign, multiple outlets reported Thursday.

The move could indicate that Flynn's legal team either is cooperating with Mueller's investigators or is negotiating to do so.

In large criminal investigations, defense lawyers routinely share information with each other. But it can become unethical to continue such communication if one of the potential targets is looking to negotiate a deal with prosecutors.

The Associated Press reported that a lawyer for Flynn communicated the decision to Trump's legal team this week.

The decision was first reported by The New York Times, in a story that cited four anonymous sources. According to the Times, Trump's attorneys have been bracing for Flynn to be indicted in recent weeks.

Flynn was forced to resign as national security adviser in February after White House officials concluded that he had misled them about the nature of his contacts during the transition period with the Russian ambassador to the United States.

He was interviewed by the FBI in January about his communications with the ambassador, Sergey Kislyak. The deputy attorney general at the time, Sally Yates, soon advised White House officials that their public assertions that Flynn had not discussed sanctions with Kislyak were incorrect and that Flynn was therefore in a compromised position.

Flynn was facing a Justice Department investigation over his foreign business dealings even before Mueller was appointed as special counsel in May. Mueller has since inherited that investigation.

Flynn, a prominent Trump backer on the campaign trail, has been a key figure in Mueller's probe and of particular interest to Trump. Former FBI Director James Comey, for instance, said that Trump encouraged him to end an FBI investigation into Flynn during a private Oval Office meeting in February.

In addition to scrutinizing Flynn's contacts with Russia during the transition and campaign, Mueller has been investigating the retired U.S. Army lieutenant general's role in $530,000 worth of lobbying work his now-defunct firm performed for a Turkish businessman during the final months of the 2016 presidential campaign.

The lobbying campaign sought to gather derogatory information on Fethullah Gulen, a Turkish cleric and green-card holder living in Pennsylvania. Turkish President Recep Tayyip Erdogan has accused Gulen of being behind a botched coup and has sought his extradition. Gulen has denied the allegations, and U.S. officials have rebuffed Turkey's extradition demands, citing a lack of evidence.

Flynn and his firm, Flynn Intel Group, carried out the lobbying and research work for several months, meeting with officials from the U.S. and Turkish governments. Flynn also published an op-ed on Election Day in The Hill newspaper, parroting many of the Turkish government's talking points about Gulen. At the time, neither Flynn nor his company was registered with the Justice Department to represent Turkish interests.

Soon after the publication of the op-ed, the Justice Department began investigating Flynn's lobbying work, and in March, he registered with the department as a foreign agent. In federal filings, Flynn acknowledged the work could have benefited the government of Turkey.

Since then, FBI agents working for Mueller have been investigating whether the Turkish government was directing the lobbying work and not a private company owned by a Turkish businessman, Ekim Alptekin, as Flynn's firm has contended. FBI agents have also been asking about Flynn's business partner, Bijan Kian, who served on Trump's presidential transition, and his son, Michael Flynn Jr., who worked for his father as part of the lobbying campaign. Flynn Jr. also was a near constant presence around his father during the Trump campaign and presidential transition period.

Mueller announced his first charges in the investigation last month, including the guilty plea of a foreign-policy adviser to the campaign, George Papadopoulos, and the indictments of former campaign chairman Paul Manafort and business associate Rick Gates.

Click for more from The New York Times.

The Associated Press contributed to this report.