An administration official confirmed that Flynn was interviewed by the FBI after he became national security adviser regarding his conversations with Russia’s ambassador to the US, Sergey Kislyak, prior to Trump’s inauguration.

These developments contributed to a sense of an administration back on its heels amid questions about its handling of a range of issues, including high-level diplomatic contacts with Russia and a North Korean missile launch.

Democrats, too, stepped up their attacks -- eager to turn the questions from Flynn to Trump himself, over what he knew and when about Flynn’s contacts with the Russians.

White House Press Secretary Sean Spicer defended the administration’s actions, saying that Flynn hadn’t violated any laws.

“The issue pure and simple came down to a matter of trust,” Spicer said at a news briefing Tuesday. “That’s why the president asked for his resignation and he got it.”

Spicer said that the White House had been “reviewing and evaluating” Flynn’s situation “for a few weeks trying to ascertain the truth,” the first time the administration had made any such admission. Trump told reporters on board Air Force One on Friday that he was unaware of a report that the Department of Justice had warned the White House about Flynn’s contacts with Kislyak and that he would “look into that.”

The Justice Department informed the White House last month that Flynn had discussed US sanctions with the Russian envoy and misled officials about the conversation, according to a US law enforcement official with knowledge of the matter. That warning was delivered to the White House counsel’s office by Sally Yates, the acting attorney general, the official said.

Spicer said on Tuesday that the warning arrived on January 26, and that Trump was briefed on it “immediately.” He questioned why the Justice Department waited to provide the information 11 days after Vice President Mike Pence defended Flynn in an appearance on CBS’s “Face the Nation.” In the interview, Pence asserted that the national security adviser did not discuss sanctions with Kislyak.

“Where was the Department of Justice?” Spicer said.

Trump fired Yates on January 30 after she said she wouldn’t defend his executive order barring entry to the US by people from seven predominantly Muslim nations.

Corker, who Trump at one point weighed as a possible secretary of state, said that in the wake of the Flynn revelations, an ongoing Senate Intelligence Committee investigation of Russia’s actions is no longer sufficient. Flynn should testify as part of a broader examination, said Corker of Tennessee.

“I think there needs to be fulsome investigation on all angles relative to nefarious activities that were taking place with Russia, beginning in March but even going back before that time,” Corker said. He said Flynn’s resignation “heightens” the need for Republican leaders to conduct an expanded probe, although he stopped short of endorsing an independent commission as Democrats have demanded.

At the same time, a number of other Republicans, including the chairman of the House Intelligence Committee, are still downplaying the need to investigate Flynn and said any probe should instead be focused on news leaks about Flynn’s phone call, echoing Trump on the issue.

Devin Nunes, a California Republican, said the leaks are “absolutely” the most troubling part of the episode. “We don’t even know this is true,” he said. “We’re going off press reports. So we want to get to the bottom of it.”

But McCain said in a statement that Flynn’s White House exit “raises further questions about the Trump administration’s intentions toward Vladimir Putin’s Russia, including statements by the president suggesting moral equivalence between the United States and Russia despite its invasion of Ukraine, annexation of Crimea, threats to our NATO allies, and attempted interference in American elections.”

Graham told CNN Tuesday that tough questions remain about whether anyone else in the White House knew about Flynn’s conversation with Russia’s ambassador shortly after President Barack Obama announced a series of sanctions against Russia ahead of Trump’s inauguration.

Flynn in his resignation letter said he had “inadvertently” misinformed Pence and Trump about discussing sanctions during his talk with the official.

“I think most Americans have a right to know whether or not this was a General Flynn rogue manoeuvre or was he basically speaking for somebody else in the White House,” Graham said. He also said lawmakers should have access to transcripts of Flynn’s conversations.

House Speaker Paul Ryan urged all sides to give the administration time to explain more about what transpired.

“I think it’s really important that as soon as they realised that they were being misled by the national security adviser, they asked for his resignation,” the Wisconsin Republican told reporters Tuesday. “I can’t speak to the rest of the circumstances. I think we need to get all of that information before we pre-judge anything.”

Senate Majority Leader Mitch McConnell of Kentucky said the Intelligence panel is already looking at questions of Russian involvement in the US election and added that it’s “highly likely that they would want to look at this episode as well.”

Democrats in both chambers said the matter underscores the need for a broader investigation of Russia’s activities that would be akin to the outside bipartisan commission that examined the September 11 attacks on New York and Washington.

“If the speaker is unwilling to support a full congressional investigation, then he should get out of the way and allow an independent commission to look into the matter,” Representative Adam Schiff of California, the top Democrat on the House Intelligence Committee, said in a statement. “Russia is a large and growing threat to the United States and liberal democracy around the world.”

So far, Republican leaders have said that the Senate Intelligence Committee will continue to lead the main probe into any contacts between presidential campaigns and Russian officials. The panel announced its probe weeks ago, backed up by subpoena power. Panel Chairman Richard Burr, a North Carolina Republican, said Tuesday that the panel will conduct “active oversight” on the Flynn issue and that he’s inquiring about any transcripts of Flynn’s conversations.

“I can’t verify the facts in the stories but I’ll go where intelligence and the agencies lead us,” Burr said.

The intelligence panel’s top Democrat, Mark Warner of Virginia, said he and Burr will meet later Tuesday to discuss the parameters of their investigation. Warner said he views the Flynn resignation as “part of what’s going to be a much larger investigation,” and that Flynn should testify “the sooner the better” as part of it.

But Democrat Chris Murphy of Connecticut dismissed the Senate Intelligence probe as “just an effort to bury this” and that he would prefer a joint committee with the Intelligence, Armed Services and Foreign Relations panels.

He said the situation is getting more serious “by the hour.”

“The White House knew that Flynn had lied and they didn’t do anything about it until they got caught,” Murphy said.

Eliot Engel, the top Democrat on the House Foreign Affairs Committee, was even blunter.

“I cannot remember in my lifetime another crisis, and that includes Watergate, that’s more serious than this one is right now,” he said at a news conference.