Donald Trump’s national security adviser has resigned his position just three weeks into the new administration, after admitting he misled Vice President Mike Pence and discussed sanctions with Russia before Trump took office.

Instead of damping-down the controversy, Michael Flynn’s resignation has instead raised more questions about how much Trump knew about his conversations with Russia and has led to calls for a deeper investigation into the administration’s links with the Kremlin.

A forlorn Flynn was seen entering the Oval Office at 8:20 p.m. Monday, and soon after the White House distributed his resignation letter, which said: “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.”

Flynn’s resignation came just hours after a report in the Washington Post revealed that the Justice Department had briefed Trump late last month about the issue, suggesting that Flynn could be vulnerable to Russian blackmail. Democrats are now asking who else within the White House could be seen as a security risk.

The White House has named retired Lt. Gen. Joseph Kellogg as interim national security adviser while it works to appoint a permanent replacement.

Flynn spoke to Russia’s U.S. ambassador Sergey Kislyak both before and after Trump’s election win last November, but it was a call on Dec. 29 that brought matters to a head. During that conversation, Flynn and Kislyak discussed the sanctions recently imposed by Barack Obama on Russia over its alleged interference in the U.S. election.

While Flynn didn’t explicitly say the sanctions would be reversed, he did counsel Kislyak that Russian President Vladimir Putin should not overreact, giving the impression that the sanctions could be reversed after Trump was inaugurated.

While the discussions may have violated the Logan Act, it was the subsequent denials by the White House that sanctions were ever discussed that led to Flynn’s resignation. Specifically, multiple sources say Flynn’s misleading the vice president on this matter was the final straw.

According to sources speaking to the New York Times, Pence was said to be “incensed” by Flynn’s misinformation, as well as his lack of contrition. Flynn is reported to have blamed his faulty memory rather than accept responsibility and apologize fully.

This isn’t the first time Flynn has misled Pence. Late last year Pence was asked seven times by CNN’s Jake Tapper if he had requested a security clearance for his son, Michael G. Flynn, who had been spreading the Pizzagate conspiracy theory on social media. Pence had been briefed by Flynn that clearance was not requested, when in fact it had been.

Three main candidates are emerging as replacements for Flynn:

• Keith Kellogg — The man holding the position temporarily is in the running to get the job full-time — though he’s seen as an outsider. Like Flynn, Kellogg is a three-star former lieutenant general and had been working as the national security council’s chief of staff in the Trump administration.

• Vice Admiral Robert Harward — The former deputy to current Defense Secretary James Mattis, Harward is thought by some to be the leading candidate. An ex-Navy Seal, Harward knows the White House well, having worked there with the National Security Council during the George W. Bush administration.

• David Petraeus — The former CIA director who led U.S. forces in Iraq and Afghanistan was already due to meet with Trump this week. While he is still widely respected in national security and intelligence circles, Petraeus is still on probation having pleaded guilty to passing classified information to his biographer. That probation will expire in a matter of weeks, but it is unclear if Petraeus would get security clearance until then.

Nothing. During a press conference with Canadian Prime Minister Justin Trudeau on Monday, questions about Flynn were ignored. While Trump’s personal Twitter account posted four messages on Monday, all were from an iPhone, which is understood to be used by aides to the president, who still prefers to use an Android device.

Prior to Flynn handing in his resignation letter Monday evening, White House adviser Kellyanne Conway said Trump had “full confidence” in Flynn, though just an hour later Press Secretary Sean Spicer contradicted this, saying, “The president is evaluating the situation.”

According to sources speaking to the New York Times, Trump’s chief adviser, Steve Bannon, had been pushing for Flynn’s resignation since Friday, a day after the initial report emerged revealing Flynn had discussed sanctions with Kislyak.

Despite the revelations in the media, and Flynn’s eventual admission of guilt, the Kremlin has consistently said there was no talk of sanctions during the call in December. On Tuesday morning, several Russian lawmakers jumped to Flynn’s defense.

Konstantin Kosachev, head of the foreign affairs committees in the upper house of Russia’s parliament, wrote on Facebook: “To force the resignation of the national security adviser for contacts with the Russian ambassador (normal diplomatic practice) is not even paranoia, but something immeasurably worse.”

Kosachev continued: “Either Trump has failed to gain his desired independence and is being cornered consistently (and not without success), or Russophobia has infected even the new administration, from top to bottom.”

Flynn has previous links with Russia, having appeared alongside Putin during the 10-year anniversary of state-run television station RT in late 2015. Just last month, Democrats called for an investigation into whether Flynn had violated Pentagon policy by receiving payment for a speech he gave at a banquet there.

Hours before Flynn resigned on Monday, the Washington Post revealed that the Justice Department had raised concerns with the Trump administration in late January that Flynn had misled the vice president.

No action was taken at the time, raising the idea that Flynn was not forced to resign for lying to Pence but because the indiscretion became public knowledge.

Democratic lawmakers are now calling for a full briefing from the Justice Department and the FBI, which is facing growing calls to conduct an investigation into the links between the Trump administration and Russia.

“We need to know who else within the White House is a current and ongoing risk to our national security,” Rep. John Conyers and Rep. Elijah Cummings said in a statement. Others are asking what information Trump had, and when he had it. Sen. Adam Schiff tweeted: “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.”

Early Tuesday morning, Hillary Clinton weighed in on the scandal, tweeting about the perils of fake news: