President Donald Trump asked FBI chief James Comey to end an inquiry into links between his ex-national security adviser and Russia, US media report.
"I hope you can let this go," Mr Trump reportedly told Mr Comey after a White House meeting in February, according to a memo written by the ex-FBI director.
The memo was written immediately after the meeting, a day after Michael Flynn resigned, according to US media.
The White House has denied the report in a statement.
"The president has never asked Mr Comey or anyone else to end any investigation, including any investigation involving General Flynn," it said.
Mr Flynn was forced out in February after he misled the vice-president about his conversations with Russia's ambassador before Mr Trump took office.
The latest Russian twist, first reported by the New York Times, comes a week after Mr Trump fired Mr Comey over his handling of the FBI investigation into Hillary Clinton's use of a private email server while at the state department.
Mr Comey's dismissal sent shockwaves through Washington, with critics accusing the president of trying to thwart the FBI investigation into Russia's alleged interference in the US election and any Moscow ties to Trump associates.
Mr Comey reportedly wrote a memo following a meeting with the president on 14 February that revealed Mr Trump had asked him to close an investigation into Mr Flynn's actions.
He reportedly shared the memo with top FBI associates.
"I hope you can see your way clear to letting this go, to letting Flynn go," the president told Mr Comey, according to the memo. "He is a good guy."
Mr Comey did not respond to his request, according to the memo, but replied: "I agree he is a good guy."
In response to the report, a White House official pointed out that acting FBI director Andrew McCabe had testified last week that there had been "no effort to impede our investigation to date".
The "I" word - impeachment - has already been broached by politicians as moderate as independent Senator Angus King of Maine. If this were a Democratic-controlled House of Representatives, articles of impeachment would likely be in the drafting process.
Republicans still call the shots in Congress, however, and it's a significant leap to get them to abandon the Trump presidency and any hope of advancing their agenda for the foreseeable future.
But some, like Senator John McCain - who said this has become a scandal of "Watergate size and scale" - are clearly wavering.
The former Republican presidential nominee is a bit of a wild card, of course.
For the rank-and-file to turn on the president will require them to admit their complicity in a failed presidency.
Mr Flynn's departure in February came months after suspicions were raised among intelligence officials.
He resigned as White House national security adviser after just 23 days on the job over revelations that he had discussed lifting sanctions on Moscow with Russian ambassador Sergei Kislyak, before Mr Trump was sworn in.
It is illegal for private citizens to conduct US diplomacy.
Since Mr Flynn stepped down, the Pentagon has launched an investigation into whether he failed to disclose payments from Russian and Turkish lobbyists for speeches and consulting work.
Mr Flynn's Russian ties are under investigation by the FBI and the House and Senate Intelligence Committees, as part of wider inquiries into claims Moscow sought to tip the election in favour of Mr Trump.
Contacts between Russia and members of the president's campaign team are also under scrutiny.
Mr Comey's sacking on 9 May fuelled claims that Mr Trump may have been trying to cover up his associates' Moscow ties.
Republican Jason Chaffetz, who chairs the House Oversight Committee, has written to the FBI demanding all correspondence relating to communications between Mr Comey and the president. He set a deadline of 24 May.
Mr Chaffetz's demand for the memo was backed by House Speaker Paul Ryan, although Mr Ryan's deputy Kevin McCarthy said there were a "a lot of allegations and... small truth".
Top Democratic Senator Chuck Schumer said he was "shaken" by the New York Times report. "The country is being tested in unprecedented ways."
Some politicians and commentators have mentioned the O-word and even the I-word - impeachment.
Adam Schiff, the highest ranked Democrat on the House Intelligence Committee, said this intervention by Mr Trump, if correct, amounted to "interference or obstruction of the investigation".
Senator John MCain reportedly said at a dinner that the Trump scandals had now reached "Watergate size".
And David Gergen, who has advised several presidents, said: "I think we're in impeachment territory now."
When Senator Angus King, an independent, was asked on CNN if this was getting closer to a possible impeachment, he said: "I have to say yes, simply because obstruction of justice is such a serious offence."
The key legal statute is 18 US Code Section 1512, which contains a broad definition allowing charges to be brought against someone who "obstructs, influences, or impedes any official proceeding, or attempts to do so".
Section 1512 requires a person not only to attempt to obstruct justice but to do it with "corrupt" intent, and legal experts have told the Washington Post that is not clear in this case.