Michael Flynn "has a story to tell," the fired U.S. national security adviser's lawyer promised back in March, hinting at a trove of insider information relevant to an investigation of links between Russia and Donald Trump's 2016 presidential campaign.

Now it appears that Flynn might already be divulging what he knows, and likely opening a line of communication with special counsel Robert Mueller's investigators. The dead giveaway? Flynn's team has stopped sharing info with Trump's attorneys.

The revelation that Flynn's lawyers severed those communications about the Trump-Russia investigation was first reported by the New York Times on Thursday.

The legal manoeuvre, while not proving they have flipped and decided to co-operate with Mueller's investigators, "is the clearest indication we've seen to date that Flynn is co-operating with prosecutors," said former federal prosecutor Barak Cohen.

Trump Russia Rallying Cry

In this June 21 photo, special counsel Robert Mueller, left, departs after a closed-door meeting with members of the Senate Judiciary Committee about Russian meddling in the 2016 election and possible connection to the Trump campaign. (J. Scott Applewhite/Associated Press)

If confirmed, legal scholars say collaboration between Flynn and Mueller would be the biggest news in the Trump-Russia probe so far.

Cohen believes Flynn's legal team has likely been speaking to the government "for a while," and that a plea agreement is "imminent."

Given the criminal exposure Flynn is believed to have — for initially failing to disclose $530,000 US in payments from the Turkish government and potentially violating the Foreign Agents Registration Act — there's incentive for him to play ball. He may also want to shield his son from indictment for his lobbying work on behalf of Turkey's authoritarian regime.

Mueller v. Trump

George Papadopolous, former foreign policy adviser with the 2016 Trump campaign, is now a co-operating witness in special counsel Mueller's Russia investigation. (Courtesy Linkedln)

Flipping Flynn could help build a case against Mueller's likely "primary target" — Trump himself, said Seth Abramson, a legal analyst and prominent Twitter user known for lashing out against Trump.

"Not because Trump's charges would be the largest charges, but because he's the most important person in the hierarchy," Abramson said.

Flynn would be able to provide a long view of Trump's relationship with Russia, particularly as he was involved in both the campaign and part of the early administration.

USA-TRUMP/BANNON

U.S. President Donald Trump is joined by former chief of staff Reince Priebus, centre, Vice-President Mike Pence, former senior adviser Steve Bannon, former communications director Sean Spicer and former national security adviser Michael Flynn, right, as he speaks by phone with Russian President Vladimir Putin. (Jonathan Ernst/Reuters)

Signs of co-operation

His attorneys were in discussions with the House and Senate intelligence committees back in March about negotiating an immunity deal in exchange for information that might be relevant to Mueller's probe into possible Russian collusion.

At the time, lawmakers said it would be premature to accept an immunity offer. Legal analysts believe Mueller used the time in between to gather leverage on Flynn before cutting a deal.

That Flynn has not communicated publicly for months suggested he was already co-operating, Cohen noted.

Defence lawyers with different clients sometimes enter into "joint-defence agreements" with one another if they share a common legal interest, in order to keep each other updated and to strategize without betraying attorney-client privilege. Continuing with such information-sharing can be deemed unethical for lawyers when a potential target is negotiating with prosecutors, giving rise to a conflict of interest.

Which is why legal experts suggest that Flynn's legal split from working with Trump's lawyers likely means Flynn is now working with Mueller.

Trump meets with Lavrov, Kislyak

Seemingly unconcerned by the optics, Trump jokes with Russian Foreign Minister Sergei Lavrov, left, and Russian ambassador to the U.S. Sergei Kislyak in the Oval Office the day after he fired FBI director James Comey, who was leading the Russia investigation. (Russian Foreign Ministry/EPA)

Typically, Cohen said, the parties might continue to engage in joint defence, even while talking with prosecutors representing the government, "until the prosecutors signal that co-operation may be jeopardized if the target of the investigation keeps talking to other targets."

For his part, Abramson argues the Trump-Flynn legal information-swapping was ethically dubious from the get-go. He cited a message that Flynn told supporters he received from Trump in April — "stay strong" — after Flynn was fired in February for lying to the vice-president about meeting the Russian ambassador.

A month after the message from Trump, Flynn refused to hand over documents subpoenaed by a Senate panel.

"Witness tampering," Abramson suggested.

The 'motherlode' of information

"What you want from Flynn is confirmation, whether through documentation or testimony, that it was in fact the deliberate plan for the Trump campaign to communicate they were going to unilaterally drop sanctions on Russia if Trump won the election," he said.

He added that Flynn's testimony would also be key to establishing whether the president meets a legal threshold of having a "high likelihood" of knowledge that Russia was committing computer crimes.

"Flynn can tell investigators more about what Trump was [allegedly] doing really behind the scenes with respect to Russia than any witness Mueller currently has ready and steady access to," he said. That includes George Papadopoulos, the Trump campaign foreign policy adviser who pleaded guilty last month to lying to federal agents about his attempts to communicate with Russia.

Flipping Flynn could yield "the motherlode" of information, said former federal prosecutor Mark Osler.

He speculated Flynn might still be at the first step towards cutting a deal — arranging a "proffer," in which prosecutors decide whether a defendant "is shooting straight" and is suitable for a co-operation arrangement.

If a deal proceeds, not only can Flynn "tell the whole story from the campaign into the whole administration," Osler said, but Flynn is also valuable for corroborating information about meetings he attended while serving as the national security adviser and in his campaign role.

Flynn's possible co-operation could also help to define who in the chaotic early days of the Trump administration called the shots, whether behind the scenes or at the forefront.

"Prosecutors love charts," Osler said. "Especially given that these prosecutors, many come out of working cases on corporations and the mob, two areas where structure would be very important."

In a Twitter thread, former federal prosecutor Renato Mariotti reacted that the development is "shocking" because it suggests Flynn no longer believes the president will pardon him or his son.

"If pardons are off the table, co-operation is likely the right move for Flynn," he wrote.