Special cousel Robert Mueller, who is leading the investigation into claims of Russian meddling in the US election, has reportedly empanelled a grand jury.
US media reports suggest Mr Mueller's inquiry has taken the first step towards possible criminal charges.
According to Reuters, the jury has issued subpoenas over a June 2016 meeting between US President Donald Trump's son and a Russian lawyer.
The president has poured scorn on any suggestion his team colluded with the Kremlin to beat Hillary Clinton.
In the US, grand juries are set up to consider whether evidence in any case is strong enough to issue indictments for a criminal trial.
They do not decide the innocence or guilt of a potential defendant. The jury is panel of ordinary citizens, and the empanellment also allows a prosecutor to issue subpoenas or legal writ to obtain documents or compel witness testimony under oath.
Thursday's reports suggest former FBI director Mr Mueller's investigation is focusing on 39-year-old Donald Trump Jr's June 2016 meeting at Trump Tower with a Russian lawyer.
According to Reuters, the special counsel is examining whether anyone affiliated with the Trump campaign encouraged the Russians to start releasing material about the Clinton campaign.
A source told the news agency the president himself was not currently under investigation.
But Mr Mueller was seeking to determine whether Mr Trump knew of his son's meeting before it happened, or if he was briefed on it afterwards, the source said.
At a rally in Huntington, West Virginia, on Thursday evening, Mr Trump said the allegations were a "hoax" that were "demeaning to our country".
"The Russia story is a total fabrication," he said. "It's just an excuse for the greatest loss in the history of American politics, that's all it is."
The crowd went wild as he continued: "What the prosecutor should be looking at are Hillary Clinton's 33,000 deleted emails."
"Most people know there were no Russians in our campaign," he added. "There never were. We didn't win because of Russia, we won because of you, that I can tell you."
Mr Trump's high-powered legal team fielding questions on the Russia inquiry said there was no reason to believe the president himself was under investigation.
"The White House favours anything that accelerates the conclusion of his [the president's] work fairly," a lawyer appointed last month as White House special counsel, Ty Cobb, said.
"The White House is committed to fully co-operating with Mr Mueller."
Earlier on Thursday, the US Senate introduced two separate cross-party bills designed to limit the Trump administration's ability to fire Mr Mueller.
The measures were submitted amid concern the president might dismiss Mr Mueller, as he fired former FBI director James Comey in May, citing the Russia inquiry in his decision.
The US first son's emails, which he personally released to the public, showed he was told the meeting would yield damaging material on Mrs Clinton, provided by the Kremlin.
Mr Trump and his aides have dismissed the encounter as "opposition research" which happens in any political campaign.
Mr Trump Jr's initial, misleading statement - that the meeting was about Russian adoptions - was issued with the president's advice.
The US intelligence community, including the CIA and NSA, has determined that Russia sought to boost Mr Trump's chances of victory in November 2016 presidential election, which Moscow denies.
Mr Trump has at times expressed doubt about the determination made by his own intelligence agencies.
Mr Mueller was appointed in May by the deputy attorney general of the Department of Justice.
Several congressional inquiries are examining the same allegations.