Story highlights It was the first time that Trump had explicitly tied the Russia probe to his rationale for firing Comey

The interview on Thursday capped a week of contradictory statements on Comey

(CNN) President Donald Trump has said he was thinking of "this Russia thing" when he decided to fire FBI Director James Comey, who had been leading the bureau's investigation into alleged Russian meddling in the 2016 election.

Trump told NBC News that he was frustrated by the ongoing investigation and believed it was motivated by Democrats' fury at losing the election.

It was the first time that Trump had explicitly tied the Russia probe to his rationale for firing Comey. It was one of a series of explanations for dismissing the FBI director, some of which directly contradict those offered earlier by top White House officials.

Trump told NBC's Lester Holt: "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, it's an excuse by the Democrats for having lost an election that they should have won'."

Week of contradictions