Michael Flynn arriving before a joint news conference between Canadian Prime Minister Justin Trudeau and US President Donald Trump at the White House on Monday. REUTERS/Carlos Barria Democratic lawmakers say there are still many unanswered questions after Trump's national security adviser, Michael Flynn, resigned on Monday evening.

Several have asked whether Flynn's apparent close communications with Russia could have compromised parts of the Trump administration.

"Flynn's resignation is a good 1st step. If colluding with Russia on sanctions is unacceptable, what about colluding with Russia on the campaign," Rep. Jamie Raskin of Maryland said in a message posted on Twitter.

A tweet sent from the Twitter account of former Rep. John Dingell of Michigan said: "Flynn's resignation CANNOT be end of the story. Who talked to who? When? Who knew? When? How far up did it go?"

Sen. Chris Murphy of Connecticut said: "Flynn's departure only matters if 1) he's followed by a mainstream, experienced replacement, 2) we get to bottom of the Trump/Russia story."

"General Flynn's resignation is a step in the right direction. But many questions remain," Rep. Joaquin Castro of Texas said Monday night.

A January assessment from Sally Yates, the acting US attorney general at the time, first reported by The Washington Post on Monday night suggested that Flynn, who had contact with Russia's ambassador to the US, may be subject to blackmail from Russia.

Both the director of national intelligence and the CIA director at the time, James Clapper and John Brennan, concurred, according to The Post, which cited an anonymous official as saying they feared "Flynn had put himself in a compromising position" with Russia.

Trump talked to Russian President Vladimir Putin on the phone about "mutually beneficial" trade and security, the Kremlin has said. Associated Press/Andrew Harnik

The Trump-Russia story

That assessment is an extension of a long-playing narrative surrounding President Donald Trump, who has often expressed an affinity for Russian President Vladimir Putin.

Trump made clear during his election campaign that he revered Putin's leadership of Russia, and he made a direct public appeal to the Kremlin during a news conference in July asking it to find damaging information on his Democrat opponent, Hillary Clinton.

A classified dossier on Trump originally compiled as political opposition research, which had been making the rounds in Washington since last year, contained unverified details of supposed associations between Trump and Russia. US officials on Friday said some elements in the 35-page document had indeed proved accurate.

Trump and his surrogates have called the document "fake and fictitious."

Business Insider's Natasha Bertrand reported on Saturday that the timeline of Trump's ties with Russia in that dossier line up with allegations of conspiracy and misconduct. The dossier is still being investigated.