WASHINGTON — President Trump offered a new version of his decision to fire James B. Comey, saying on Thursday that he would have dismissed the F.B.I. director regardless of whether the attorney general and his deputy recommended it.

It was just the latest in a series of statements, some of them contradictory, to whiplash Washington over 48 hours that began with Mr. Comey’s firing on Tuesday evening. And it was unusually harsh: Mr. Trump castigated Mr. Comey as “a showboat” and “a grandstander,” suggesting that his issues with the F.B.I. director went beyond any previously stated concerns.

Mr. Trump said on Thursday that he had not relied solely on the advice from the Justice Department’s top two leaders in making his decision. And, for the first time, he explicitly referenced the F.B.I.’s investigation into his administration’s ties to Russia in defending Mr. Comey’s firing.

“And in fact, when I decided to just do it, I said to myself, I said, ‘You know, this Russia thing with Trump and Russia is a made-up story,’” Mr. Trump told Lester Holt of NBC News. “It’s an excuse by the Democrats for having lost an election that they should have won.”