Special counsel Robert Mueller is using a grand jury in Washington as part of an investigation into potential co-ordination between the Trump campaign and Russia.
The use of a grand jury, a standard prosecution tool in criminal investigations, suggests that Mr Mueller and his team of investigators are likely to hear from witnesses and demand documents in the coming weeks and months.
The Wall Street Journal first reported the use of a grand jury.
Grand juries are common vehicles to subpoena witnesses and records and to present evidence, though they do not suggest any criminal charges are near or will necessarily be sought.
It was not immediately clear how or whether the Washington grand jury was connected to the work of a separate one in Alexandria, Virginia. That panel has been used to gather information on Michael Flynn, Donald Trump's former national security adviser.
A spokesman for Mr Mueller's team did not return an email seeking comment.
Mr Mueller's reliance on a grand jury is the "logical next step in this investigation" given that it is the traditional method for prosecutors to gather evidence, said Washington defence lawyer Jacob Frenkel.
"The use of the grand jury neither escalates, nor establishes a timeline for, the investigation," he added.
Meanwhile, lawyers for Mr Trump said they were unaware of the existence of a grand jury and had no information to suggest the president himself was under federal investigation.
"With respect to the news of the federal grand jury, I have no reason to believe that the president is under investigation," defence lawyer John Dowd said.
Ty Cobb, special counsel to the president, said he was not aware Mr Mueller had started using a new grand jury.
"Grand jury matters are typically secret," Mr Cobb said.
"The White House favours anything that accelerates the conclusion of his work fairly ... The White House is committed to fully co-operating with Mr. Mueller."
Mr Mueller was appointed special counsel in May by the justice department following the firing by Mr Trump of FBI director James Comey.
He has since assembled a team of more than a dozen investigators, including current and former justice department prosecutors with experience in international bribery, organised crime and financial fraud.
News of the grand jury came as senators introduced two bipartisan bills aimed at protecting Mr Mueller from being fired by Mr Trump, with both parties signalling resistance to any White House effort to derail the investigation into Russian meddling in last year's election.
Mr Trump's defence team has been looking into potential conflicts of interest among members of Mr Mueller's team, such as past political contributions to Democrats including Hillary Clinton.
Mr Trump has warned that any effort by Mr Mueller to look into his finances would fall outside the scope of Mr Mueller's appointment.
Under the regulations, Deputy Attorney General Rod Rosenstein is the government official empowered to fire Mr Mueller under limited circumstances, such as conflict of interest and dereliction of duty.
Mr Rosenstein, who appointed Mr Mueller, has publicly said he has seen no basis for firing him.
Mr Trump's lawyer Jay Sekulow said: "The president is not thinking about firing Robert Mueller so the speculation that's out there is just incorrect."
He also downplayed the significance of the grand jury, calling it "a standard operating procedure when you've got a situation like this".