Administration aides said on Tuesday that senior officials at the White House and the Justice Department had been charged with building a case to justify Mr. Comey’s firing since at least last week, and that Attorney General Jeff Sessions had been tasked with coming up with reasons to fire him…

But the final straw appeared to be Mr. Comey’s appearance at a Capitol Hill hearing last week, in which he testified that while he was “mildly nauseous” to think he might have influenced the outcome of the presidential election, he had no regrets about his handling of the Clinton investigation…

Mr. Trump’s disdain for Mr. Comey intensified at the beginning of March, when the president wrote Twitter posts accusing Mr. Obama of wiretapping his telephone at Trump Tower.

The next morning, word spread quickly that Mr. Comey wanted the Justice Department to issue a statement saying that he had no evidence to support the president’s accusation, although no such statement was released.