President Trump told Russian officials in the Oval Office this month that firing the F.B.I. director, James B. Comey, had relieved “great pressure” on him, according to a document summarizing the meeting.“I just fired the head of the F.B.I. He was crazy, a real nut job,” Mr. Trump said, according to the document, which was read to The New York Times by an American official. “I faced great pressure because of Russia. That’s taken off.”Mr. Trump added, “I’m not under investigation.”The conversation, during a May 10 meeting — the day after he fired Mr. Comey — reinforces the notion that Mr. Trump dismissed him primarily because of the bureau’s investigation into possible collusion between his campaign and Russian operatives. Mr. Trump said as much in one televised interview, but the White House has offered changing justifications for the firing.The White House document that contained Mr. Trump’s comments was based on notes taken from inside the Oval Office and has been circulated as the official account of the meeting. One official read quotations to The Times, and a second official confirmed the broad outlines of the discussion.