U.S. President Trump speaks about tax reform during a visit to Loren Cook Company in Springfield Thomson Reuters A top Democratic member of the House Intelligence Committee says the committee wants to examine any documentation related to President Donald Trump's decision to fire FBI Director James Comey in May, including a letter the president wrote at the time outlining his reasons for wanting Comey gone.

"We want to see anything memorialized around" the firing, Rep. Eric Swalwell told Business Insider when asked if the committee wants to see a copy of the letter.

Rep. Adam Schiff, the top Democrat on the House Intelligence Committee, told CNN on Sunday that the committee had not yet seen the letter. But he said "it’s probably past time for our committee to subpoena the White House to make sure we get all relevant documents."

Schiff's Republican counterpart, Mike Conaway, who took over committee's Russia investigation after Rep. Devin Nunes stepped aside in April, did not respond to request for comment.

The letter, obtained by FBI special counsel Robert Mueller shortly after it was written, is significant because it provides a snapshot of the president's state of mind before he fired Comey.

In that sense, it could be instructive in determining whether Trump fired Comey because he thought he was truly unfit to lead the FBI, or if he dismissed him in the hopes that the Russia probe — which Comey was leading at the time — would go away.

According to White House special counsel Ty Cobb, Trump decided he wanted Comey gone shortly after he testified before the Senate Judiciary Committee on May 3. Comey reiterated during that public appearance that some of Trump's associates were still under investigation, and he said it made him "mildly nauseous" to think that his handling of the Hillary Clinton email probe "might have had some impact on the election."

Comey had not allowed the White House to review his testimony, which Trump and his aides considered "an act of insubordination," according to Reuters. The Times echoed that report, saying Trump was broadly irked by his inability to gain assurances of loyalty from Comey.

Reports have also suggested that Trump was annoyed with Comey for implying that the election was somehow swayed by the director's controversial decision to tell Congress that he was reexamining Clinton's emails 11 days before the election.

As such, Trump wrote a letter with one of his top policy advisers, Stephen Miller, outlining why he thought Comey was unfit to lead the FBI and expressing frustration with the fact that Comey would not confirm publicly that he was not under investigation.

Cobb told Business Insider last weekend that there was "little if any pushback" from within the White House to the president's "memorandum," which he described as "exonerating."

But the letter — described as a "rant" by the Washington Post and a "screed" by the New York Times — was never released, and White House counsel Don McGahn was reportedly alarmed when it was presented to him. McGahn heavily edited it and sent it back to Miller, according to the Times, and it was then sent over to the DOJ.

The advice McGahn gave to Trump may prove pivotal in the obstruction of justice case that FBI special counsel Robert Mueller is reportedly building against the president. Cobb told BI that the White House gave Mueller the letter voluntarily, but did not clarify whether they handed over the original, unedited version or the one marked up by McGahn.

Trump's legal team has sought to fend off a potential obstruction charge by arguing that the president has the authority to fire whomever he wants and that Comey is an unreliable witness, according to the Wall Street Journal.

But someone in Trump's orbit — if not McGahn, then someone else with sway over the president — was clearly not confident that the president's rationale for firing Comey could stand on its own.

The letter was never sent, and Trump instead wrote a short addendum to memos written by Attorney General Jeff Sessions and Deputy Attorney General Rod Rosenstein outlining Comey's purported missteps. Neither of the memos mentioned the Russia investigation.