Robert Mueller. AP Photo/Andrew Harnik

President Trump's drafted letter laying out his reasons for firing FBI director James Comey could give the special counsel a direct window into the president's intent when he later dismissed Comey.

The letter could also implicate top Trump aide, Stephen Miller, in Robert Mueller's obstruction-of-justice investigation.

The advice that White House counsel Don McGahn gave Trump to dissuade him from sending the letter could also prove to be a critical piece of the puzzle.

News on Friday that special counsel Robert Mueller has obtained a letter drafted by President Donald Trump that details his reasons for firing then-FBI director James Comey has likely bolstered the progress of the Russia investigation, and may have landed another close Trump confidant in its crosshairs.

Mueller was put in charge of the investigation — which is examining whether the Trump campaign colluded with Moscow during last year's presidential election — after Trump dismissed Comey in May. As part of his investigation, Mueller is also examining whether Trump obstructed justice when he fired the FBI director four months ago.

The letter Mueller is reviewing was drafted by Trump along with policy adviser Stephen Miller, and legal experts say it is possibly the most critical piece of evidence in Mueller's obstruction-of-justice case since Comey's testimony before the Senate Intelligence Committee in June, because it can give prosecutors a direct window into Trump's thinking shortly before he fired Comey.

The biggest challenge a prosecutor faces in an obstruction-of-justice case is proving corrupt intent, which is almost always difficult to establish. But Trump's letter could change the ballgame.

"The best way to prove someone's intent is through their own words and actions," former federal prosecutor Renato Mariotti told Business Insider in an interview Saturday. "Here, you have a letter that was written by Miller, at the direction of the president, that contains what the president's thoughts were at that time."

Though the letter's full contents remain unclear, The Washington Post reported that it focused on what was perhaps Trump's greatest frustration with Comey: that the FBI director did not publicly announce, when he was leading the bureau's investigation, that Trump was not personally under investigation.

Former FBI Director James Comey Drew Angerer/Getty Images "It's problematic for Trump if he fired Comey because he did not take actions in the investigation that would benefit Trump personally," Mariotti said. "That makes Mueller's case stronger."

Cornell Law School associate dean and criminal law expert Jens David Ohlin echoed that assessment.

"The draft letter is extremely relevant to Mueller's investigation because it may yield evidence about the true reason that Trump fired Comey," Ohlin said. "If Trump fired Comey to impede an investigation that might implicate his own campaign or administration, that is obstruction of justice."

Trump put the letter together shortly after Comey's May 3 testimony before the Senate Judiciary Committee, during which he defended his handling of the investigation into Hillary Clinton's use of a private email server when she was secretary of state. The president was reportedly incensed after Comey acknowledged that his October announcement that the FBI was reopening its investigation into Clinton, days before the election, could have impacted its results.

Trump's lawyer, Ty Cobb, told Business Insider in an email Saturday that the letter has long been in Mueller's possession and its existence was known both to the special counsel's team, as well as to the Department of Justice, "which has had a copy since the day it was first discussed within the White House." He added there was "little, IF ANY, objection within the White House" to the letter, and that it focused primarily on Comey's "usurpation of powers and other erratic and inexplicable conduct."

The long weekend during which Trump drafted the letter at his Bedminster golf club began on Thursday, May 4, The New York Times reported on Friday. Deputy attorney general Rod Rosenstein was given a copy of Trump's draft letter on Monday, May 8, and then proceeded to write a separate memo as to why Comey should be fired.

Another Trump confidant comes under scrutiny

Stephen Miller tapes Sunday show interviews from the White House. REUTERS/Joshua Roberts

The letter also implicates Miller, who The Post said acted as a "stenographer" for Trump in writing the letter.

Miller, an ally of the recently ousted chief strategist Steve Bannon, has emerged in recent months as a Trump loyalist within the administration.

Given his role in the matter, Miller will likely be, at the very least, a witness in Mueller's investigation. Other possible witnesses include Trump's daughter, Ivanka, and her husband, Jared Kushner, who were both with Trump at his Bedminster golf club when he drafted the letter during a weekend in early May.

If Miller acted primarily as a transcriber, he could have a smaller part in the investigation. However, "if he was actively working with the president to plan how they could derail or kill the Russia investigation," Mariotti said, "that could present legal problems for Miller."

Ohlin added that Miller and anyone else involved in Comey's firing — or drafting the letter — may be accessories or co-conspirators to that crime as well.

The question then becomes, Mariotti added, "whether there was an agreement between Miller and the president to obstruct justice." If that were the case, it could amount to conspiracy, he said.

Another way the adviser could be implicated in the investigation is if, for example, the president was acting in a way to obstruct justice, and Miller knew about that and tried to do what he could to help Trump succeed. If that were the case, Miller could have been aiding and abetting a crime.

Mariotti said those two possibilities are likely the biggest potential sources of criminal liability for Miller.

The letter, as a whole, is a crucial part of the Russia controversy because it "goes directly to the biggest issue at question — what Trump's intent was as to the Russia investigation," Mariotti said.

Trump's best defense would likely be that the draft letter did not reflect his true thinking on the subject, and that's why never sent it, Ohlin said.

He added, however, that he didn't believe the argument would hold much water because "it seems more likely that the draft letter reflected his true thinking, but then was edited down for other reasons."

Though the White House initially said that Trump fired Comey based entirely on Rosenstein's and Attorney General Jeff Sessions' recommendations, Trump later said he had already decided to fire Comey, and that Rosenstein's recommendation sealed the deal.

His explanation changed again later on, when he admitted to NBC News' Lester Holt that he had fired Comey because of "this Russia thing," and that he was going to dismiss the FBI director regardless of Rosenstein's input.

President Donald Trump looks at Finnish President Sauli Niinisto during a joint news conference in the East Room of the White House, Monday, Aug. 28, 2017, in Washington. AP Photo/Alex Brandon

And as far as that goes, White House counsel Don McGahn's conversation with Trump when he advised him against sending the letter could be another key piece of the puzzle.

"We don't know exactly what McGahn said, but the mere fact that he put a stop to that letter is another piece of evidence that Mueller could use to say, 'Donald Trump was warned by the White House counsel that this was a problematic step and decided to do it anyway,'" Mariotti told Business Insider on Saturday morning, and later spoke about on Twitter.

The substance of what McGahn told Trump is important — and there's no guarantee that it could be withheld as privileged information.

The reason is that a federal court of appeals ruled in 1998, at the height of the Monica Lewinsky scandal, that deputy White House counsel Bruce Lindsey had to submit to the special prosecutor's questions about President Bill Clinton's relationship with Lewinsky. In that case, the court held that there is no attorney-client privilege between a government lawyer and a government employee in response to a grand jury inquiry.

If that ruling holds as it relates to the obstruction-of-justice investigation, it's possible the public will eventually hear what McGahn told the president. "If he said anything along the lines of, 'There's potential criminal liability if you shut down this investigation,' that would be extraordinarily powerful evidence against Trump," Mariotti said.

In that case, McGahn's advice to Trump could possibly become as important as Trump's state of mind when he crafted the letter.