Special counsel Robert Mueller, who is leading the investigation into Russia's alleged meddling in the 2016 election, has a copy of a draft letter written by President Trump and a top White House aide detailing why he would terminate former FBI Director James Comey, according to a report.

Trump and Stephen Miller, a White House senior policy adviser, wrote the letter from Trump's golf club in Bedminster, N.J., in May, the New York Times reported. It's unknown what the draft letter said, but sources told the Washington Post it did not focus explicitly on the Russia probe.

The president showed top White House aides a copy of the letter during a meeting in the Oval Office on May 8, the day before Comey was fired, the Washington Post reported.

The letter was several pages and included a long list of complaints Trump had about Comey, including that he refused to say publicly he wasn't under investigation by the FBI, the Washington Post reported Friday.

Comey ultimately said in congressional testimony he privately told Trump the FBI wasn't investigating him as part of its probe into Russian meddling in the 2016 election.

Sources told the New York Times that White House counsel Donald McGahn took issue with parts of the letter and successfully stopped the president from sending it to Comey.

Instead, Comey was sent a different letter from Trump on May 9 that included a memo from Deputy Attorney General Rod Rosenstein. That memo primarily focused on Comey's handling of the FBI's investigation into Hillary Clinton's use of a private email server.

Attorney General Jeff Sessions also sent a letter to Trump, which included Rosenstein's memo and recommended the former FBI director be terminated.

In his short letter to Comey, Trump said new leadership was needed at the FBI to restore "public trust and confidence in its vital law enforcement mission," and said he decided to fire the former director based on Sessions and Rosenstein's recommendations.

"While I greatly appreciate you informing me, on three separate occassions, that I am not under investigation, I nevertheless concur with the judgment of the Department of Justice that you are not able to effectively lead the bureau," Trump wrote.

Mueller received a copy of Trump and Miller's drafted letter from the Justice Department in recent weeks.

Ty Cobb, a lawyer with the White House, declined to discuss the letter with the New York Times, but said, "To the extent the special prosecutor is interested in these matters, we will be fully transparent with him."

The New York Times said the letter originally drafted by Miller and Trump may provide the best explanation for why Trump decided to fire Comey. However, it's unknown how much of that explanation addresses the FBI's investigation into Russian meddling in the 2016 election, part of which looks into the ties between Trump campaign officials and Russia.