Sean Spicer, the White House press secretary, refused to “re-litigate” the delay during his daily briefing on Monday. But here are the various explanations he and other administration officials have given in the past.

Feb. 14

A day after Mr. Flynn was fired, Mr. Spicer spent most of his daily briefing responding to questions about the 18-day delay. He described the warning from Ms. Yates as a “heads-up” and repeatedly said that the information she provided had been quickly relayed to the president.

“Immediately after the Department of Justice notified the White House counsel of the situation, the White House counsel briefed the president and a small group of his senior advisers.”

But Mr. Spicer added that Mr. Trump had doubts, believing Mr. Flynn had done nothing wrong and ordering a review of the situation.

“He instinctively thought that General Flynn did not do anything wrong, and the White House counsel’s review corroborated that.” “The president asked them to commit a review of whether there was a legal situation there. That was immediately determined there wasn’t.”

On the question of why the president had not acted sooner, Mr. Spicer suggested that doing so would have violated Mr. Flynn’s right to due process.

Feb. 16

Two days after Mr. Spicer’s comments, the president was asked about Mr. Flynn during a 77-minute news conference in the East Room of the White House. Mr. Trump called Mr. Flynn “a fine person” but said he was “not happy” with the way his former national security adviser had provided information to Vice President Mike Pence.

“You know, he was just doing his job. The thing is, he didn’t tell our vice president properly, and then he said he didn’t remember. So either way, it wasn’t very satisfactory to me.”

The president did not directly address the delay in firing Mr. Flynn, and instead repeatedly mentioned his concerns about the leaking of classified information to reporters in the room.

“The first thing I thought of when I heard about it is, how does the press get this information that’s classified? How do they do it? You know why? Because it’s an illegal process, and the press should be ashamed of themselves.”

Feb. 19

Reince Priebus, the White House chief of staff, was asked directly about the 18-day delay on NBC’s “Meet the Press” that Sunday. He described the message from Ms. Yates as a “heads-up” about Mr. Flynn that “something wasn’t adding up with his story.”

“The fact was, it turned more or less into a conversation about whether or not he was being honest with us and the vice president. And the president asked for his resignation, and we got it.”

Mr. Priebus described that conversation inside the West Wing as a kind of slow-motion evolution in the thinking of White House officials, who waited for a review of the situation from the counsel’s office.

“And then, some time after that, the legal department came back and said that they didn’t see anything wrong with what was actually said. But then we started thinking about whether or not Michael Flynn was being straight with us. And that’s when we started asking a lot of questions and sort of deposing Michael Flynn, and figuring out what he knew or what he didn’t know.”

March 6

By early March, the explanation from the White House had become shorter. In a daily briefing, Mr. Spicer responded to a question about aides misleading the president by noting that Mr. Flynn had “not been straight” with the vice president.

“He asked him to resign. I think the president dealt with it.”

March 10

Four days later, Mr. Spicer answered a question about ethical standards at the White House by referring again to the president’s decision to fire Mr. Flynn. He again declined to mention the 18-day delay between when the president was informed of Mr. Flynn’s deceptions and the decision to let him go.