Donald Trump is increasingly vulnerable to charges of obstructing justice and may have inadvertently confessed following the prosecution of his former senior aide Michael Flynn, according to legal experts and senior Democrats.

Flynn plea deal increases exposure of senior Trump transition team members Read more

The US president said in a tweet on Saturday that he fired Flynn as national security adviser in February “because he lied to the vice-president and the FBI” about his discussions with Russia’s ambassador to the US last December. Flynn pleaded guilty in court on Friday to lying to FBI agents.

This would mean that Trump knew Flynn had committed a serious crime when, according to the former FBI director James Comey, the president asked Comey the next day to halt an FBI investigation into Flynn. On Sunday Trump, who later fired Comey, again denied making such a request.

Multiple legal analysts and critics of the president said Trump’s remark was the clearest indication so far that he has tried to obstruct the various inquiries into possible collusion between Moscow and his presidential campaign, leaving him vulnerable to criminal charges.

“He could be tweeting himself into an obstruction of justice conviction,” said Richard Painter, a former ethics counsel to the George W Bush administration.

“That’s a confession of deliberate, corrupt obstruction of justice,” said Laurence Tribe, a professor in constitutional law at Harvard University.

Moving to limit the potential damage, one of Trump’s attorneys, John Dowd, claimed he had written the tweet, which he described as “sloppy”.

Dianne Feinstein of California, the most senior Democrat on the Senate judiciary committee, which is conducting its own Trump-Russia inquiry, said on Sunday that “what we’re beginning to see is the putting together of a case of obstruction of justice” against the president.

He could be tweeting himself into an obstruction of justice conviction Richard Painter

“I see it most importantly in what happened with the firing of Director Comey, and it is my belief that that is directly because he did not agree to lift the cloud of the Russia investigation,” Feinstein told NBC’s Meet the Press. “That’s obstruction of justice.”

The latest frenzy of activity from the White House followed Flynn’s guilty plea and agreement to cooperate with a sprawling criminal investigation by Robert Mueller, the special counsel, who has already indicted two other Trump campaign officials for alleged crimes and accepted a guilty plea from a third.

Mueller’s decision to agree a deal with Flynn indicates that the former national security adviser holds evidence that could help Mueller prosecute even more senior figures for even more serious crimes, according to legal analysts.

Experts told the Guardian the wording of Flynn’s plea agreement also left open the possibility that Flynn had already worn a wire or otherwise recorded conversations with other associates of Trump who are under investigation.

A central unanswered question is why Flynn lied to the FBI if there was not more serious wrongdoing that he was trying to cover up. “It is a shame because his actions during the transition were lawful,” Trump said in his tweet on Saturday. “There was nothing to hide!”

Flynn falsely denied to FBI agents that he had urged Russia’s ambassador to refrain from retaliating against sanctions on Moscow imposed by Barack Obama on 29 December, in response to Russian meddling in the election. While it is technically illegal for a private citizen to negotiate with a foreign government in dispute with the US, no one has ever been convicted of breaking that 218-year-old law.

Documents released by Mueller on Friday said Flynn was being directed around that time by a “very senior” member of Trump’s transition team. Multiple US news outlets reported that this was Jared Kushner, Trump’s son-in-law and senior adviser.

Among issues being examined by investigators is whether any agreement existed between Russia and Trump – or senior Trump advisers – that Trump’s administration would act favourably towards the Kremlin in return for Russian assistance in the election campaign. US intelligence agencies concluded that Moscow mounted an influence campaign aimed at helping Trump defeat Hillary Clinton.

Play Video 0:52 ‘No collusion’ between presidential campaign and Russia, says Trump – video

The New York Times on Saturday published details of emails from transition team members. One national security adviser, KT McFarland, wrote that Obama’s sanctions would complicate Trump’s relations with a country “which has just thrown the USA election to him”. The Times said a White House lawyer told it McFarland had been referring to how Democrats portrayed the election result.

Donald Trump: Michael Flynn's actions during transition were lawful Read more

Other comments made by Trump could also put him in jeopardy of obstruction charges. Several Republican senators have said Trump asked them to stop congressional inquiries into Russia’s interference in the 2016 election.

And after stating publicly that he had fired Comey for other reasons, Trump privately cited Comey’s pursuit of the Russia investigation as the reason, according to a leaked White House transcript.

“I faced great pressure because of Russia,” Trump said. “Now, that’s taken off.”

On Sunday Mike Rogers, a Republican former Michigan congressman who served on Trump’s transition team, said Trump’s actions around the firings of Flynn and Comey were likely to be central to the outcome of the Russia inquiry.

“I do believe James Comey,” Rogers said on CNN’s State of the Union. “I think James Comey is a very good man. I think his testimony is going to be credible.”

In a series of increasingly wild tweets on Sunday, Trump attacked the FBI and ABC News, which suspended veteran journalist Brian Ross for an error in his reporting on the prosecution of Flynn on Friday.

The president also seized on reports that Mueller fired an FBI agent from his team for having sent text messages critical of Trump during the election campaign.

“After years of Comey, with the phony and dishonest Clinton investigation (and more), running the FBI, its reputation is in Tatters – worst in History! But fear not, we will bring it back to greatness,” Trump said in one tweet.