Retired Lt. Gen. Mike Flynn resigned as the White House national security adviser Monday night after less than a month in the position.

The controversy around Flynn’s secret talks with the Russian ambassador is the first to draw real blood from the Trump White House, which has seen a series of cascading crises during its first three and a half weeks in office.

“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. I have sincerely apologized to the president and the vice president, and they have accepted my apology,” wrote Flynn in his resignation letter Monday evening. “I am tendering my resignation, honored to have served our nation and the American people in such a distinguished way.”

Flynn’s resignation will embolden lawmakers on the left and right who have called for deeper probes into the ties between the Trump White House and the Russian government. Multiple committees in the House and Senate are already in various stages of investigations, but some lawmakers have called for a 9/11-style independent commission to look into the matter. At the very least, Flynn’s departure will create pressure to broaden the existing Russia investigations.

“Michael Flynn is only resigning because he got caught by press reports revealing improper contacts with Russia,” Rep. Eric Swalwell, the top Democrat on the subcommittee overseeing the CIA, told The Daily Beast. “We must learn the full extent of any prior and existing personal, financial and political relationship between Donald Trump and the Russian government.”

In the interim, Retired Lt. Gen. Joseph Kellogg will serve as acting national security adviser, the White House announced. Kellogg, like Flynn, has spent decades in the military. He served two tours during the Vietnam War, then later was the commander of the famed 82nd Airborne Division and a director under the Joint Chiefs of Staff.

A senior administration official said retired Navy SEAL Vice Admiral Robert Harward is the leading candidate to replace Flynn, ahead of the other man under consideration, retired Gen. David Petraeus. Harward worked closely with Secretary of Defense Jim Mattis throughout his career, including serving as his deputy when Mattis headed Central Command. Harward has worked as an ABC News consultant and a defense contractor based in the Gulf since his retirement.

The official confirmed that the Justice Department had warned the White House that Flynn was a Russian blackmail risk, “which is preposterous,” the official said, speaking on condition of anonymity to discuss Flynn’s resignation.

The official blamed many of the leaks on former Obama administration officials and holdover-officials from the previous administration still working at outside law enforcement and national security agencies.

“These assholes have impunity to leak classified documents to destroy an innocent man... they get away with it,” said the frustrated official of Flynn’s departure. “They shiv you with one hand and plug you with the other.”

President Trump, who weighed in on Twitter as the morning shows dissected Flynn’s demise, also questioned the origin of the media reports that had fatally undermined his national security adviser. “The real story here is why are there so many illegal leaks coming out of Washington?” he wrote.

Democratic lawmakers insist that Flynn’s resignation does not answer their pressing questions—including the full extent of his contact with the Russian government, and whether he was acting on the wishes of the president.

“Flynn’s departure does not end questions over his contacts with the Russians, which have been alleged to have begun well before Dec. 29. These alleged contacts and any others the Trump campaign may have had with the Kremlin are the subject of the House Intelligence Committee’s ongoing investigation,” said Rep. Adam Schiff, the top Democrat on the House Permanent Select Committee on Intelligence. “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.”

The strongest support for Flynn came from Moscow on Tuesday morning. Konstantin Kosachev, chairman of the Russian foreign affairs committee, said he did not desrve to lose his job and lashed out at Trump for allowing Flynn to resign. “Either Trump hasn’t found the necessary independence and he’s been driven into a corner,” he said. “Or Russophobia has permeated the new administration from top to bottom.”

Flynn’s resignation comes at the end of his 24th day in office—a day of contradictory messages from the Trump administration over Flynn’s status—and just hours after The Washington Post reported that the White House had been warned weeks earlier that Flynn’s misleading statements about his conversations with the Russian ambassador could make him a target for blackmail.

On Monday afternoon, presidential counselor Kellyanne Conway told MSNBC that the president had “full confidence” in Flynn. Yet Flynn’s resignation would be tendered before the day was even over.

Flynn appeared in the front row at Trump’s Monday news conference with Canadian Prime Minister Justin Trudeau—just in time to have to sit through a television correspondent’s live comments questioning whether he’d be fired. But the two American reporters Trump called on did not ask him about Flynn.

That awkward public appearance followed a weekend of ominous silence from the White House, after the admission by Trump officials that Flynn did discuss Obama administration sanctions with Russian Ambassador Sergei Kislyak before Trump took office. That’s a potential violation of U.S law, first reported by The Washington Post.

Worse, Flynn told Vice President Mike Pence that he didn’t discuss sanctions, a claim Pence then repeated in defense of Flynn in multiple interviews.

The Trump administration officials delivering that news to the press Friday looked sucker-punched and disbelieving, even as they spoke the information out loud, on the condition of anonymity to discuss the sensitive matter.

Perhaps they shouldn’t have been so surprised. According to The Washington Post, Acting Attorney General Sally Yates told Team Trump in late January that Flynn had misled his superiors—and maybe even opened himself up to blackmail. The directors of national intelligence and the Central Intelligence Agency shared these concerns. “We’ve been working on this for weeks,” a senior administration official told the paper.

But then how to explain this past, surreal weekend? Flynn traveled together with Trump and Japanese Prime Minister Shinzo Abe to Trump’s Mar-a-Lago resort in Florida, even advising the two leaders about the weekend’s North Korean missile launch over Saturday evening’s dinner. (That event is itself drawing fire from the intelligence world because Trump and Abe conferred with Flynn and other experts while at the dinner table, as waiters served salads and guests snapped selfies.)

Throughout, journalists in Florida and Washington, D.C., were trying to find out if the retired three-star would survive.

In what is arguably the most combative White House in ages when it comes to defending its people or its record, no one would answer the question “Does President Trump have confidence in his national security adviser?” Most glaringly, White House policy maestro Stephen Miller, who was rolled out on the Sunday shows to defend Trump’s executive order on refugees, refused to be drawn on Flynn’s plight.

“It’s a sensitive matter,” Miller said, telling NBC host Chuck Todd that it wasn’t his place to comment.

The controversy starkly revealed the divide among three key centers of power in the White House: Counselor to the President Steve Bannon’s senior staff in the Strategic Initiatives Group, pointedly staying out of the fray; Pence and Chief of Staff Reince Priebus’s traditional GOP quarter, struggling to determine whether it would be more damaging to let Flynn stay and look the other way, but invite congressional censure; and Flynn’s loyal tribe atop the NSC, resenting the hits their boss was taking but realizing this was, in soccer terms, an own goal.

The Trump administration’s detractors in Congress began to circle. On Friday, several Democratic lawmakers called on Flynn to step down from his position. And on Monday, all 15 Democrats on the House Oversight Committee called on Chairman Jason Chaffetz, a Republican from Utah, to investigate Flynn’s ties to Russia—or step aside and allow the full committee to vote on whether to go ahead with such a probe.

Tension mounted Monday among Flynn’s friends, as well. His detractors took bets on when he would get booted out the door—and started trading names of those who might succeed him. Retired four-star Gen. David Petraeus was a popular choice. But in the end, another retired general officer, Lt. Gen. Keith Kellogg, would become the interim national security adviser.

Flynn exited with a campaign-minded flourish. He had gained notoriety on the political scene with ‘Lock Her Up’ chants during the Republican National Convention. He would exit the White House with similar slogans.

“I know with the strong leadership of President Donald J. Trump and Vice President Mike Pence and the superb team they are assembling, this team will go down in history as one of the greatest presidencies in U.S. history, and I firmly believe the American people will be well served as they all work together to help Make America Great Again,” read the ending of Flynn’s resignation letter.

He signed it: “Michael T. Flynn, LTG (Ret). / Assistant to the President / National Security Advisor.”

UPDATE 12:34 a.m.: The story has been updated throughout to reflect Flynn’s resignation.