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Former Trump lawyer Michael Cohen uncorked many disturbing allegations in his seven hours of congressional testimony this week, but only one that could be crucial to the future of the special counsel Robert Mueller’s investigation.

Cohen testified he was in the room when Republican strategist Roger Stone called President Donald Trump and told him about an impending WikiLeaks release of stolen Democratic National Committee emails. Stone and Trump have not only denied that claim — but also done so in a coordinated way that may shape what Mueller ultimately does.

The consequences here are simple.

If Cohen lied to Congress, he could be indicted for it and the world would soon find out.

If Cohen told the truth, then Trump is in trouble.

The allegation could be pivotal because Trump and Stone have been united in denying it, not only in public statements, but also in legally binding assurances to the government.

Lying in public is legal. Lying to the government is a crime. That is why this part of Cohen’s allegation poses such serious legal risks to both Stone and Trump.

Unfortunately for the White House, Cohen’s account has extra corroboration, which provides more supporting evidence than his other new claims that could be dismissed as a “he said/he said” dispute.

Here is why. Cohen testified he “was in Mr. Trump's office” in July 2016, when Stone called Trump to warn about an impending “WikiLeaks drop of emails.”

That matches the broader evidence Mueller has on Stone, Trump's longtime confidant. The special counsel’s indictment states that "around June and July 2016," Stone told senior Trump campaign officials about the email dump. In legal terms, Mueller believes he has the evidence to back up that claim, which he must do to prove Stone lied about it.

We don’t know everything Stone told the House in closed testimony, but we do know about a key exchange, because in order to indict Stone, Mueller had to publicly file it. Stone was asked, “Did you discuss your conversations with the [WikiLeaks] intermediary with anyone involved in the Trump campaign?”

Stone replied, “I did not.”

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That is part of what got Stone indicted. Mueller supports the charge with language in the indictment that, in light of Cohen’s new testimony, is even more interesting: “Stone spoke to multiple individuals involved in the Trump Campaign about what he claimed to have learned.”

One of those multiple individuals could be Trump. After all, though Cohen’s testimony is new to us, Mueller’s team has been sifting through it for months. Or Mueller may be referring to Stone’s contacts with other “individuals” in the Trump campaign, to avoid relying on Cohen as a witness at Stone’s trial. (These kinds of indictments deliberately avoid naming the “individuals.”)

Either way, there’s one sign Mueller thinks this line of inquiry is vital: It was reportedly on his list of questions for the president.

Mueller had this question for Trump, according to The New York Times: “What did you know about communication between Roger Stone, his associates, Julian Assange, or WikiLeaks?”

In public, Trump flatly denied Stone told him anything about WikiLeaks.

Now in fairness, if that is false, it is always possible Trump’s lawyers got him to be more accurate (or evasive) when privately answering Mueller. No one knows because the special counsel has kept Trump’s answers secret.

There are two hints, however, suggesting Trump may have given the same denial to Mueller.

First, CNN reported that’s exactly what the president did.

“Trump told special counsel Robert Mueller in writing that Roger Stone did not tell him about WikiLeaks,” the news organization reported in November, citing two sources. (NBC News has not confirmed that account.)

The second hint involves a Trump loyalist who is standing firm while others flip. He’s the self-proclaimed “dirty trickster” with a Nixon back tattoo: Roger Stone.

Even after his indictment, Stone made a point of proclaiming that his story matches Trump’s secret written answers to Mueller. This was important, and suspicious, even before Cohen told his WikiLeaks story to Congress.

Stone, who insists he’s not in touch with Trump about this, still decided to tell the world his denial matches one of the most closely guarded secrets in the Mueller probe — what Trump told the special counsel about intelligence that could relate to collusion.

In an exchange with Fox News’ Tucker Carlson, which could draw scrutiny from Mueller’s office, Stone said after his indictment that he hasn’t discussed these topics with Trump — but that somehow he knew his denial matched Trump’s secret answers to Mueller.

“When the president answered the written interrogatories, he correctly and honestly said ‘Roger Stone and I never discussed this’ — and we never did,” Stone said.

Think about it: Stone knows what he did, but how could he be so confident about what Mueller actually asked Trump? Or how Trump responded?

Lying to the special counsel is a crime. And Mueller has already indicted others for doing it.

It sounds suspicious on its own. Then factor in that Mueller has amassed evidence to conclude Stone is lying. So if Stone and Trump’s answers do match, that same evidence could be marshaled against the president.

Lying to the special counsel is a crime. And Mueller has already indicted others for doing it.

In the special case of presidential oversight, any crime committed while a president is in office is more significant than a crime committed before taking office. The Constitution provides a structure that may hold presidents more accountable for lies a president tells prosecutors than for other lies, such as lying to the public, or statements made before assuming office.

If Cohen is telling the truth and Mueller has the evidence, this is the crux of why Cohen’s account could be troubling for Trump.

Finally, it is worth appreciating how this legal predicament is based on the relatively thin clues in the public record. There are two more unknowns that may ultimately focus or resolve the situation.

What other secret evidence do investigators have? And, if the Trump campaign did nothing wrong and Stone merely touted WikiLeaks access he did not have, why not just admit that and move on? Instead, we are witnessing the spectacle of Stone and Trump loudly denying an alleged 2016 phone call that simply echoed their public messaging at the time — the view that WikiLeaks was a “treasure trove” and Stone had an inside line. There’s no legal reason to “cover up” that conversation, if that’s all it was.

Ari Melber, an attorney, is MSNBC’s chief legal correspondent and host of The Beat with Ari Melber.