Earlier this year, when reporters were sniffing around Donald Trump Jr.’s now-infamous June 2016 meeting with a Russian attorney, some people inside the Trump camp wanted to come clean: to issue a statement saying that Trump Jr.’s meeting was about getting Russian dirt on Hillary Clinton.

The president, however, disagreed.

During a flight on Air Force One in July, he personally dictated a statement to White House spokesperson Hope Hicks saying that the meeting “primarily discussed a program about the adoption of Russian children.”

That statement, released on July 8 and attributed to Donald Trump Jr., was quickly shown to be verifiably false. And several reports released in the past week indicate that special counsel Robert Mueller is investigating just how it was drafted. The Daily Beast, for example, reported on September 8 that Hicks herself will almost certainly be questioned regarding the statement’s composition. The Washington Post reported on the same day that five other Trump aides would also be questioned, and that the July 8 statement would likely be a focus of some of those conversations as well.

Lying to the press is not a crime. But there’s a reason Mueller seems to be focusing so much on this statement anyway: The circumstances around its drafting may be a uniquely good source of evidence of obstruction of justice, committed either by Trump or by his close associates. It’s the kind of evidence that could be used to support impeachment charges against the president and criminal charges against some of his associates.

“The fact that the president was so intensely involved in it, and felt a desire to be intensely involved in crafting it, raises questions about his intent and knowledge,” Renato Mariotti, a partner at Thompson Coburn LLP and former federal prosecutor, says. “That could make it easier for Mueller to meet his burden of proof as to an obstruction of justice charge.”

The reason this matters so much: it speaks to Trump’s intent

To prove obstruction of justice, the law requires prosecutors to prove that their intentions were corrupt. But intent is hard to prove in a criminal case. It requires prosecutors to show what someone was thinking, and people often don’t leave verifiable proof of their thought processes lying around. The conversations surrounding the July 8 letter’s drafting, however, create a situation where intent could actually be uncovered.

During the Air Force One flight, Trump and his top associates sat down and discussed how to handle the Trump Jr. Russia meeting. The specific reasoning that led them to decide on lying as a PR strategy could end up pointing to evidence of intent to obstruct justice by the president and/or his staff.

“It helps to establish a pattern of deception,” says Ryan Goodman, a former Defense Department special counsel and current editor of the legal site Just Security. “If Mueller’s point is that the president has been trying to either interfere with the investigation into any potential ties between the Trump campaign with the Russians, or he himself might be implicated ... engaging in this form of deception could be significant and could be entered into a trial.”

This what makes Hicks, who was officially appointed White House communications director in mid-September, such a significant figure. She reportedly argued in favor of coming clean about the meeting’s true purpose in July and was overruled. This means she could speak to the reasoning that was used to overrule her — reasoning that could, arguably, reveal criminal intent. So too could any other Trump aides present during the discussion.

It’s unlikely that anything uncovered by Mueller would lead him to file criminal charges against Trump.

The Office of Legal Counsel, whose guidelines govern the Justice Department, has advised since 2000 that sitting presidents cannot be indicted. Legal experts disagree about whether this guidance is correct, but both Mariotti and Goodman believe that Mueller would be unlikely to try to push it. Instead, they say, any untoward behavior by Trump uncovered during an investigation of the July 8 statement could be handed over to Congress for impeachment proceedings. So even though Mueller isn’t likely to indict Trump himself, he’ll continue to investigate potential criminal wrongdoing by the president.

When it comes to Trump associates, like Donald Trump Jr. or Jared Kushner, the indictments are a very real possibility. This meeting could uncover either evidence of their intent or even evidence of new crimes, like lying to Congress.

“It puts Donald Trump Jr. into great jeopardy: According to news reports, Don Jr. told the Senate Judiciary Committee that he did not inform his father about the meeting. That would be a materially false statement,” Goodman says.

None of this means that the July 8 meeting is the smoking gun, that it will provide the single piece of evidence necessary for Trump Jr.’s indictment.

What it does show, though, is that Mueller is systematically compiling evidence that could be used to put together criminal indictments or fuel an impeachment drive. He’s doing what federal investigators do when they’re getting a serious case: building up as much evidence as possible in order to guarantee a conviction if they go to trial. That should very much worry the Trump team.

“Proving that someone obstructed justice, particularly in a very unusual fact pattern like this, is a challenge,” Mariotti says. “Anything like this that makes his job significantly easier is important, because it suggests that something that seems on its face very difficult to do is closer to the realm of possibility.”