WASHINGTON — Lawyers for Michael T. Flynn, President Trump’s former national security adviser, notified the president’s legal team in recent days that they could no longer discuss the special counsel’s investigation, according to four people involved in the case — an indication that Mr. Flynn is cooperating with prosecutors or negotiating a deal.
Mr. Flynn’s lawyers had been sharing information with Mr. Trump’s lawyers about the investigation by the special counsel, Robert S. Mueller III, who is examining whether anyone around Mr. Trump was involved in Russian efforts to undermine Hillary Clinton’s presidential campaign.
That agreement has been terminated, the four people said. Defense lawyers frequently share information during investigations, but they must stop when doing so would pose a conflict of interest. It is unethical for lawyers to work together when one client is cooperating with prosecutors and another is still under investigation.
The notification alone does not prove that Mr. Flynn is cooperating with Mr. Mueller. Some lawyers withdraw from information-sharing arrangements as soon as they begin negotiating with prosecutors. And such negotiations sometimes fall apart.
Still, the notification led Mr. Trump’s lawyers to believe that Mr. Flynn — who, along with his son, is seen as having significant criminal exposure — has, at the least, begun discussions with Mr. Mueller about cooperating.
Lawyers for Mr. Flynn and Mr. Trump declined to comment. The four people briefed on the matter spoke on the condition of anonymity because they were not authorized to discuss it publicly.
A deal with Mr. Flynn would give Mr. Mueller a behind-the-scenes look at the Trump campaign and the early tumultuous weeks of the administration. Mr. Flynn was an early and important adviser to Mr. Trump, an architect of Mr. Trump’s populist “America first” platform and an advocate of closer ties with Russia.
His ties to Russia predated the campaign — he sat with President Vladimir V. Putin at a 2015 event in Moscow — and he was a point person on the transition team for dealing with Russia.
The White House had been bracing for charges against Mr. Flynn in recent weeks, particularly after charges were filed against three other former Trump associates: Paul Manafort, his campaign chairman; Rick Gates, a campaign aide; and George Papadopoulos, a foreign policy adviser.
But none of those men match Mr. Flynn in stature, or in his significance to Mr. Trump. A retired three-star general, Mr. Flynn was an early supporter of Mr. Trump’s and a valued surrogate for a candidate who had no…