A letter authored by President Trump and a senior adviser detailing the president's reasoning for firing FBI Director James Comey reportedly complained that the former top cop wouldn't publicly say that Trump was not personally under investigation.

The Washington Post reported Friday that the letter, which was never delivered to Comey, is currently under review by special counsel Robert Mueller, who is leading the federal investigation into the Trump campaign's possible ties to Moscow and collusion to influence the outcome of the presidential election.

Comey confirmed in testimony before a Senate panel in June that he had privately told Trump that he was not personally under investigation as part of the FBI's Russia probe. Comey said he didn't make the disclosure public because the status could change down the road.

Trump fired Comey in May, sending him a shorter letter that cited the recommendations of Attorney General Jeff Sessions Jefferson (Jeff) Beauregard SessionsTrump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status White House officials voted by show of hands on 2018 family separations: report MORE and his deputy, Rod Rosenstein, that Comey be terminated over his handling of the FBI investigation into Hillary Clinton Hillary Diane Rodham ClintonWhat Senate Republicans have said about election-year Supreme Court vacancies Bipartisan praise pours in after Ginsburg's death Trump carries on with rally, unaware of Ginsburg's death MORE's use of a private email server while secretary of State.

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Trump drafted the initial letter with Stephen Miller, a senior policy adviser and speechwriter, in May before sharing it with senior aides, according to multiple reports.

Trump ultimately decided not to send the letter after his aides urged caution in doing so, the Post reported.

According to The New York Times, which first reported that Mueller had obtained Trump's initial letter, the draft was blocked by White House counsel Don McGahn.

Days after firing Comey, who was at the time leading the FBI's Russia probe, Trump acknowledged that he had decided to oust him before receiving a recommendation from the Justice Department.