U.S. President Trump arrives at Morristown municipal airport for a weekend at the Trump National Golf Club in Bedminster ahead of next week's United Nations General Assembly Thomson Reuters FBI special counsel Robert Mueller wants to know what President Donald Trump was thinking when he fired James Comey as FBI director and helped craft a misleading statement about his son's meeting with Russians last year at Trump Tower.

And he apparently thinks White House counsel Don McGahn has the answers.

To charge someone with obstructing justice, prosecutors have to prove that "the defendant corruptly endeavored to influence, obstruct, or impede" an investigation, according to legal and national-security experts writing for Lawfare.

That element "is the hardest to prove, because it depends on showing an improper motive," the experts said.

As such, insight into Trump's state of mind when he fired Comey and crafted the statement will be crucial as Mueller examines whether the president willingly tried to impede the Russia investigation.

"Mueller is trying to figure out Trump's state of mind in relation to obstructive acts," Andy Wright, a former associate counsel to President Barack Obama who is now a professor at Savannah Law School, said.

McGahn came under scrutiny earlier this month when The New York Times reported that he had blocked Trump from sending a letter to Comey outlining his reasons for wanting him fired.

Longtime federal prosecutor-turned defense attorney Renato Mariotti noted that at the time that that letter — which reportedly referenced Trump's displeasure with Comey's handling of the Russia probe — and McGahn's subsequent pushback could prove pivotal for Mueller's obstruction case.

Mariotti also noted that, if McGahn were called to testify before a federal grand jury, his conversations with Trump would not be protected by attorney-client privilege (a federal appeals court ruled in 1998, at the height of the Monica Lewinsky scandal, that there was no attorney-client privilege between a government lawyer and a government employee as it related to a grand-jury inquiry).

Now it appears that Mueller will look to McGahn for answers as to why Trump crafted a misleading statement about his son's meeting with two Russians — a lawyer and a lobbyist — at Trump Tower last June. That detail, revealed by the Times, appears to be the first indication that Mueller is interested in hearing from McGahn about something other than his alleged role in creating a legal justification for Trump to fire Comey.

White House press secretary Sarah Huckabee Sanders told reporters last month that "the president weighed in" on the Trump Tower meeting "as any father would based on the limited information he had."

But Mueller's interest in the initial statement — which did not mention that Trump Jr. had been offered incriminating information on Hillary Clinton in exchange for taking the meeting, or that more than one Russian attended — has reportedly been growing as he investigates whether Trump tried to cover up any interactions he or his associates had with Russians last year.

With regard to Comey's dismissal, "McGahn will be a fact witness to what Trump was saying at the time" and "can tell Mueller what concerns he raised with President Trump, which will further shed light on Trump's state of mind," Wright noted.

Trump's lawyers have argued that Comey is an unreliable witness and that Trump has the power to fire whomever he wants. But the White House has not fully explained why Trump reportedly overruled his advisers' warnings to be as transparent as possible about the Trump Tower meeting, Wright noted — or why one of his personal lawyer, Jay Sekulow, told CNN in the days after the meeting became public that Trump did not play any role in crafting his son's initial statement.

Here, too, McGahn likely has insight into Trump's state of mind around the time that he drafted the statement, making him a crucial witness as Mueller attempts to put pieces together.

"As to the narrower legal question of obstruction of justice, the president's mental state would critical," Wright said. "Did he participate in crafting a cover story to hoodwink the American people or did he have congressional and criminal investigators in mind?"

It is unclear whether Trump consulted McGahn before drafting the statement about the Trump Tower meeting. McGahn's attorney did not return a request for comment.

If McGahn was called, then he was in a position to hear Trump's thinking "and to shape that thinking" through his legal advice, Wright noted.

But if he wasn't consulted, that would likely signal to Mueller "either that Trump was profoundly unaware of the nature of his deteriorating legal environment or that he was avoiding legal advice because he did not want to be told to stand down," Wright said. "Neither option looks great."