Michael Flynn, the national security adviser, stepped down Monday after it emerged that he had misled the White House over the nature of his talks with the Russian ambassador.

Flynn initially claimed that he had not discussed sanctions during a call with Ambassador Sergei Kislyak. Vice President Mike Pence also claimed that Flynn had not had substantive discussions with the ambassador. But last week, the Washington Post cited nine sources who alleged that Flynn had indeed discussed the Obama administration’s sanctions, which were imposed in retaliation for Russian hacking during the 2016 campaign.

“Unfortunately, because of the fast pace of events, I inadvertently briefed the Vice President Elect and others with incomplete information regarding my phone calls with the Russian Ambassador,” Flynn said in a letter of resignation. “I have sincerely apologized to the President and the Vice President, and they have accepted my apology.”

The White House announced that Lt. Gen. Joseph Keith Kellogg would assume the role of national security adviser on an interim basis.

The Washington Post and other publications also reported on Monday that Sally Yates, who had been acting attorney general, informed the White House late last month that Flynn had misled administration officials about the nature of his contacts with the Russian ambassador and that Flynn could be vulnerable to Russian blackmail.

The Flynn story has been escalating for days, as the White House seemed to offer confusing messages on his fate. On MSNBC on Monday afternoon, White House Counselor Kellyanne Conway said that Trump had “full confidence” in Flynn, while spokesman Sean Spicer told reporters that the president was “evaluating the situation.”

His resignation still leaves key questions about Flynn’s interactions with Russian officials, as well as about the timing.

Rep. Adam Schiff (D-Calif.), the ranking member of the House Intelligence Committee, released a statement in which he noted that Flynn’s contacts with the Russians, as well as interactions by any others on the Trump campaign, are the focus of an ongoing committee investigation.

“Moreover, the Trump administration has yet to be forthcoming about who was aware of Flynn’s conversations with the Ambassador and whether he was acting on the instructions of the President or any other officials, or with their knowledge,” Schiff said.

CNN and MSNBC gave extensive coverage to Flynn’s resignation, while Fox News continued to air a replay of Tucker Carlson’s show.

Hillary Clinton tweeted out a comment that referred to Flynn’s resignation. It was Flynn who had engaged in a chant of “Lock Her Up!” at the Republican National Convention, referring to Clinton’s use of a private email server while she was secretary of state.

Some journalists were crediting the news media’s role in the story.