The Associated Press has not reviewed the letter.
The documents, experts believe could indicate that the special counsel is looking seriously at whether Trump committed an obstruction of justice on potentially various fronts.
That element "is the hardest to prove, because it depends on showing an improper motive", the experts said. Mr Rosenstein's letter was ultimately used as the Trump administration's public rationale for Mr Comey's firing, which was that the FBI chief had mishandled the investigation into Mrs Hillary Clinton's private e-mail server.
The president wrote the four-page letter with the help of senior White House aide Stephen Miller, the Journal reported.
"In that hypothetical, McGahn's testimony is devastating for Trump", Mariotti said.
The letter contained Trump's "unvarnished view" of why Comey should be fired.
Trump could then defend himself by saying he had no corrupt or criminal intent and was simply following McGahn's advice, Marrioti explained.
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Trump flared up again following the release of the excerpts, posting on Twitter that Comey had exonerated Clinton months before the investigation had actually ended, ascribing it to a "rigged" political system.
The contents of the original letter appear to provide the clearest rationale that Mr. Trump had for firing Mr. Comey.
Second, Trump seems to have forgotten his own White House's talking points. I have no professional, personal knowledge of it, but when you're looking at someone for something unrelated to taxes, still to get tax return information is very valuable information that tells you a lot of valuable things: "who owes them money, who they owe money to, and where they keep their money". He has said that in one of his first conversations with Attorney General Jeff Sessions, they discussed the need for new leadership at the Federal Bureau of Investigation. These officials said that Comey began to work on a statement exonerating Clinton as early as April 2016, before Clinton herself and some key aides were interviewed.
We're not sure what's in the letter, but it could factor into Mueller's investigation of whether Trump obstructed justice.
Hours later, Trump told NBC: "I was going to fire Comey - my decision. Shortly after that hearing, Mr. Rosenstein had expressed his concerns about Mr. Comey to a White House lawyer, who relayed details of the conversation to his bosses at the White House", the report said. He also said the "FBI's reputation and credibility have suffered substantial damage" under Comey.
According to the report, McGahn did warn Trump that the Russian Federation probe wouldn't go away even if he fired Comey-a point the president acknowledged but said he wanted to do it anyway. "That is true", Andy Wright, a former associate counsel to President Barack Obama who is now a professor at Savannah Law School, told Business Insider.