The investigation will enter a new phase as prosecutors led by Robert Mueller obtained a grand jury indictment against at least one person who will be charged and taken into custody, it has been revealed.
Although the nature and target of the charges has yet to be identified, the revelations triggered a guessing game among US government aides after days of trying to turn the attention away from the allegations.
And sources say the phones of President Donald Trump’s White House and personal lawyers have been ringing non-stop as they try to establish where the knife will fall in the developing scandal.
But in a scathing burst of tweets early on Sunday, Donald Trump again denounced the investigation as a “witch hunt” and repeated his denials of any collusion with Russia.
He tweeted: “The Dems are using this terrible (and bad for our country) Witch Hunt for evil politics, but the Republicans are now fighting back like never before.
“There is so much GUILT by Democrats/Clinton, and now the facts are pouring out. DO SOMETHING!
“All of this ‘Russia’ talk right when the Republicans are making their big push for historic Tax Cuts & Reform. Is this coincidental? NOT!”
A federal grand jury approved the indictment on Friday and ordered it sealed, with a source adding it could be unsealed as soon as Monday.
The indictment could mark a dramatic turn in special counsel Robert Mueller’s investigation into allegations of Russian interference in the 2016 race and any possible links with officials from President Trump’s campaign.
The Russia investigation has cast a dark shadow over the nine-month presidency after US intelligence agencies concluded in January that Moscow interfered in the election to help Trump defeat Democrat Hillary Clinton by hacking and releasing embarrassing emails to discredit her.
Mr Mueller, a former director of the Federal Bureau of Investigation, has been looking into possible links between Trump aides and foreign governments, as well as potential money laundering, tax evasion and other financial crimes, according to sources familiar with the probe.
He also is exploring whether President Trump or his aides have tried to obstruct the investigation – although the US leader has repeatedly denied allegations of collusion with the Russians.
So far investigators have interviewed former White House chief of staff Reince Priebus, former spokesman Sean Spicer and other current and former White House and campaign officials.
In July, FBI agents raided the Virginia home of Mr Trump’s former campaign manager Paul Manafort, with his previous work for a pro-Russian political party in Ukraine being investigated by Mueller’s team.
Mr Mueller also has investigated Michael Flynn, an adviser to Trump’s campaign and later his national security adviser – although he was fired from the latter post in February after misleading Vice President Mike Pence about the extent of his conversations with Russian Ambassador Sergei Kislyak last year.
The Kremlin also has denied the allegations.