But many questions remain about what charges, if any, Flynn would face and whether Mueller's prosecutors are focused on his private business dealings and truthfulness with federal agents, or if they're looking for a bigger fish like the president himself or those who remain in his inner circle.
A plea would certainly be a Washington bombshell, putting a retired U.S. Army lieutenant general and close friend of the president in a criminal courtroom and planting the sprawling investigation led by the no-nonsense former FBI director squarely in the White House.
Yet the extreme secrecy of Mueller's investigation - including the ability to keep the lid on the arrest of a Trump campaign foreign policy adviser for months - has left even those who regularly interact with his prosecutors reading tea leaves. And it's made sorting out the significance of recent events surrounding Flynn an amorphous - and at times partisan - exercise. Outside observers are urging caution in reading too much into the moves, while acknowledging that some are more significant than others.
"You get so few scraps of information that it's awfully tempting to unpack what little information you have and see what's there," said Andrew Leipold, a professor at the University of Illinois College of Law.
A critical person in Trump's campaign and national security team, Flynn was present for consequential decisions during the formative days of the administration and functioned as a main conduit for contacts with Russian officials. He could be an essential witness for Mueller, if he chose, as he investigates potential coordination between the Trump campaign and Russia.
In recent days, though, White House lawyers have downplayed the significance of Flynn's legal troubles for the president, drawing a clear line between Flynn's personal baggage and his work on the Trump campaign and the administration.
The feeling of suspense around the Mueller investigation only deepened this week with the cancellation of grand jury testimony, an ABC News report that Flynn's attorney was meeting with Mueller's team and the revelation Wednesday that Trump's son-in-law, Jared Kushner, had been questioned about Flynn earlier this month by special counsel prosecutors.
Leipold, the law professor, said he would be careful attaching too much meaning to the recently postponed grand jury testimony. But he said it was potentially a telling sign of cooperation with Mueller's team that Flynn's attorneys have broken off communication, or information-sharing, with the Trump legal team.
"It means something," he said. "If you and I are cooperating and you say all of a sudden, 'I'm not cooperating anymore,' there's probably a pretty good reason," he said.
The cutting of contact with Trump's legal team came last week after Kushner was questioned by Mueller's investigators, which occurred earlier this month.
The questioning was brief - 90 minutes or less - and tightly focused on Flynn. It was in part aimed at determining whether Kushner had any exculpatory information on Flynn, according to a person familiar with Mueller's investigation. Kushner and Flynn were both prominent figures in the Trump campaign, the presidential transition and the early days of the Trump administration.
The two also took part in discussions during the presidential transition with Sergey Kislyak, Russia's ambassador to the United States at the time, about establishing a backchannel between the two countries, a possible indication of prosecutors' interest given Mueller's mandate to probe contacts between Trump associates and the Kremlin.
Flynn was forced to resign from the White House in February after officials concluded that he had misled them about his contacts with Kislyak during the transition period. Weeks before he was fired, he was interviewed by the FBI about that communication, and former FBI Director James Comey has said Flynn was under investigation for potentially lying to federal agents about the nature of their conversation.
Mueller's grand jury had planned in coming days to hear testimony from an employee of a public relations company that worked with Flynn's firm on $530,000 worth of lobbying and investigative research for a Turkish businessman. The testimony was slated to focus on Flynn's firm's interactions with congressional staff. But it was abruptly postponed by prosecutors.
The details of Kushner's questioning and the postponement of the grand jury testimony were confirmed by people familiar with Mueller's investigation. They spoke on condition of anonymity because they weren't authorized to publicly discuss the ongoing investigation.
Asked about the meeting with Mueller, Kushner's attorney, Abbe Lowell, did not elaborate on the nature of the question, saying only in a statement his client "has voluntarily cooperated with all relevant inquiries and will continue to do so."
For his part, Flynn has stayed quiet.
His attorney, Robert Kelner, has not responded to multiple media inquiries, even as headlines about his client have piled up. Kelner also did not respond to emails, texts and calls from The Associated Press this week.
Mueller's spokesman, Peter Carr, has yet to comment on the special counsel's ongoing investigation that has now stretched into its seventh month.
Instead, the special counsel has decided to speak in indictments and plea agreements. In the meantime, Washington - and the country - wait for the next one.
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