U.S. President Donald Trump asked for and got the resignation of former National Security Advisor Michael Flynn because of an "eroding level of trust" in him and his explanations about his contacts with Russia, the White House said Tuesday.
White House spokesman Sean Spicer said, "The president concluded he no longer had trust in his national security adviser."
Flynn, a retired Army general, was ousted Monday night just three weeks into his tenure as one of Trump's top strategic advisers, a virtually unheard of quick departure for a top official at the start of an American presidency.
Spicer said Trump was pleased that Flynn had talked with Sergey Kislyak, the Russian ambassador to the United States, and concluded there was nothing legally wrong with their discussions, even though Flynn was a private citizen, not a government official, at the time.
WATCH: Spicer on what led to resignation
Misled vice president
But the White House spokesman said Trump concluded that Flynn had misled Vice President Mike Pence in January before Trump's inauguration by telling Pence that he had not discussed sanctions imposed on Russia by former President Barack Obama, who had concluded that Moscow meddled in the U.S. presidential contest in an effort to help Trump win.
Flynn acknowledged in his resignation letter that he had "inadvertently briefed" Pence and others with "incomplete information" regarding his phone calls with Kislyak in the weeks before Trump assumed power.
Even as the White House drama played out, Trump attempted to downplay its significance on Tuesday. He wrote on his Twitter account, "The real story here is why are there so many illegal leaks coming out of Washington? Will these leaks be happening as I deal on N.Korea etc?"
Key opposition Democratic lawmakers called for expanded investigations into links between Russia and Flynn and other key Trump aides, extending beyond the conclusion reached by the U.S. intelligence community last year that Russia interfered in the U.S. election.
"This. Is. Not. Normal," said Democratic Senator Elizabeth Warren, a vocal Trump critic.
WATCH: Republican House Leader Ryan on Flynn
Search for replacement
After Flynn's resignation, Trump quickly named another retired Army general, Keith Kellogg, as his acting national security adviser, but also could pick former Central Intelligence Agency chief David Petraeus or former Navy Vice Admiral Robert Harward to fill the strategic position on a permanent basis.
Flynn and Kislyak, according to widespread U.S. news accounts, discussed the sanctions Obama imposed late last year.
Pence and Trump aides, relying on information from Flynn, said publicly that Flynn had not discussed lifting the sanctions, which are still in place, once Trump was inaugurated January 20. Flynn later acknowledged the issue may have come up.
Conversations such as those between Flynn and Kislyak possibly could have been a violation of a U.S. law that prohibits private citizens from conducting diplomatic affairs with a foreign government, because Trump had yet to take office. Spicer said the White House concluded it was not a legal issue involving Flynn's contacts with Russia, but rather his misleading assertions to Pence.
Spicer said trust in Flynn "eroded to a point that we felt we had to make a change."
Justice Department warning
The U.S. Justice Department, according to The Washington Post, warned the White House last month that Flynn had so misrepresented his conversations with the Russian envoy that he might be vulnerable to blackmail by Moscow, as a result of the contradictions between the public descriptions of the calls and what intelligence officials knew based on their routine monitoring of communications by foreign officials in the United States.
Flynn, in his resignation letter, said he had apologized to both Trump and Pence, and that they had accepted his apology.
Kremlin spokesman Dmitry Peskov said Tuesday that Flynn's resignation "is the internal business of the Americans, it is the internal business of President Trump's administration. This is not our business."
But Russian lawmakers said Flynn's quick departure amounted to an attack on attempts to improve relations between Moscow and Washington.
Leonid Slutsky, who heads the Duma's foreign affairs committee, said Flynn's resignation was a "negative signal."
A member of the Duma, Konstantin Kosachev, who is chair of Russia's Foreign Affairs Committee, said, "Even a readiness to have a dialogue with Russians is seen by the hawks in Washington as a thought crime. Forcing the resignation of the national security adviser for contacts with the Russian ambassador, which is normal diplomatic practice, is not just paranoia but something immeasurably worse."
As a string of U.S. news accounts about Flynn's conversations were published, several Democratic senators called for an investigation of Flynn, while others urged Trump to fire him and for intelligence officials to review his security clearance.
Nine anonymous people described as current and former U.S. officials told the Post that Flynn and Kislyak explicitly discussed the sanctions placed on Russia by Obama after revelations that Russia had hacked into the computers of John Podesta, the campaign chief for Democrat Hillary Clinton, the former U.S. secretary of state Trump defeated in the November election.
The anti-secrecy group WikiLeaks released thousands of Podesta's emails in the weeks leading up to the election, many of them showing embarrassing, behind-the-scenes details of how Democratic operatives worked to ease Clinton's path to the Democratic presidential nomination.