Former national security adviser Michael Flynn has flipped and struck a deal to cooperate with special counsel Robert Mueller. What does that say about the direction of Mr. Mueller’s investigation into Russian attempts to influence the 2016 US presidential election?

One word provides a powerful hint. It’s near the bottom of the Statement of Offense released by Mueller’s office Friday in the Flynn case. That word is “very.” It’s used in the phrase “very senior member of the Presidential Transition Team.”

What it indicates is that Mr. Flynn will testify in court that a top transition official – “very senior” – directed him to contact officials from foreign governments, including Russia, to try and defeat a UN vote condemning Israeli settlements in December 2016.

Who was that person? The circle at the top was small, centered on President-elect Trump and members of his family. All have denied such contact. All presumably are now aware that meddling in foreign affairs while someone else remains president is, at least technically, illegal.

But the December UN vote is only one smallish item of Mueller’s interest, after all.

What the use of the word means in a larger sense is that Mueller’s investigation has entered the White House and crept close to the Oval Office itself.

With Flynn, the special counsel may have acquired a witness who can explain and tie together who knew what, and when, about the known Russia contacts of Donald Trump Jr., Trump son-in-law Jared Kushner, former campaign chief Paul Manafort, and others.

In that sense he’s a guide into unexplored territory.

“I’m pretty confident Flynn is singing like a bird to Mueller,” says Andy Wright, an associate professor at Savannah Law School and a founding editor of the legal blog Just Security.

Is Jared Kushner next?

Mr. Kushner might be the next person affected. Circumstantial evidence hints he’s the person who gave Flynn the order to try to influence the UN vote. He’s the White House official tasked with trying to negotiate a Middle East peace deal. He’s traveled often to the region. On Friday, Bloomberg News reporter Eli Lake said other former transition officials had identified Kushner as the “very senior official” in question.

Flynn, in his plea deal, also admitted he had lied to the FBI about talking with Russian Ambassador Sergei Kislyak about new US sanctions on Dec. 28 and 29, 2016. In fact, he had urged Mr. Kislyak to advocate a restrained Russian reaction to these sanctions, saying Moscow would get a better deal with the incoming Trump team.

Meddling in foreign policy via contact with a foreign power by a private citizen is illegal under a 1799 statute known as the Logan Act. But it’s a dusty statute, seldom used, and no American has ever been convicted under this law.

“I would be surprised if the first successful Logan Act prosecution is against someone in this investigation. That’s possible, but it would be a very aggressive move by Mueller,” writes Renato Mariotti, a former federal prosecutor and current Democratic candidate for Illinois attorney general, in a tweet responding to a reporter’s inquiry.

Obstruction of justice?

For the president, the biggest impact from Flynn’s flip might involve another legal avenue entirely.

“For Trump the most serious peril is obstruction of justice,” said Norman L. Eisen, former White House special counsel for ethics and government reform under President Obama, in an interview late last month.

Mueller appears to be investigating whether Trump personally obstructed justice by firing former FBI Director James Comey. According to Mr. Comey, before he was ousted from the bureau Trump asked him directly whether he could see fit to let Flynn off the hook of the Russia investigation, since he was a “good guy.” Comey gave no such indication. He lost his job.

That could look to a prosecutor as if Trump was trying to shield someone he knew had damaging information about himself or his family. In turn that could bear on his intent in dismissing Comey – a crucial question in establishing whether a particular move obstructs justice or not.

Trump also reportedly contacted Flynn after he was fired from the White House for allegedly lying to Vice President Mike Pence about his Russia contacts. The president told Flynn to “stay strong,” according to these reports. A prosecutor might interpret that as an attempt to tamper with – obstruct – the testimony of someone Trump knew might be an important witness against him.

In this context, it’s important to remember that obstruction of justice isn’t easy to prove. It’s not just about actions. It’s about state of mind – “corrupt intent,” according to lawyers. That’s something very difficult to unravel in a court of law.

One last thing the Flynn plea proves is that the Mueller investigation will have more surprising, big news days such as this in the weeks and months to come. The probe is a submarine that mostly glides below the surface.

Much missing from documents

There is a lot Mueller did not say in the court documents released as part of Flynn’s plea deal. They focused on Flynn’s lies about contacts with foreign leaders. There was nothing further about internal contacts about Russia, or knowledge of meetings about Russia, or group meetings with Russians prior to the inauguration. They shed little light on the core issue of whether the Trump campaign worked with Russia in any way during the campaign.

That could be because there was no such coordination. It could also be because the special counsel is quietly building his case.

Mueller has dozens of interview transcripts, piles of National Security Agency wiretap transcripts, tax returns, emails, and huge amounts of other material we know nothing about, says Professor Wright of Savannah Law School. Put that all together, and the full story may be much clearer than it now appears.

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“I’m sure they are sitting on mountains of that stuff,” says Wright.

Editor's note: ABC News issued a correction of its Friday special report. It now says that, according to a confidant, Flynn is prepared to testify that President-elect Trump directed him to make contact with the Russians during the transition – initially as a way to work together to fight ISIS.