The special counsel investigating claims of Russian meddling in the US election has begun using a grand jury in Washington, reports say.
The move suggests Robert Mueller may be taking a more aggressive approach to gathering data on possible collusion with Donald Trump's campaign team.
Grand juries are used to issue subpoenas to compel people to testify.
The president has again poured scorn on the inquiry, telling a rally in West Virginia it was a "total fabrication".
What is a grand jury?
In the US, grand juries are composed of members of the public who hear evidence in secret.
Prosecutors use them to gather evidence, as they can compel people to testify or hand over documentation.
They consider whether evidence is strong enough to issue indictments for a criminal trial.
The juries do not decide the innocence or guilt of a potential defendant.
How is it being used in this case?
Prosecutors have for months been using a grand jury in the Eastern District of Virginia, which had issued some subpoenas in the case, reports say.
Mr Mueller has now opted for one of the several grand juries that sit in Washington, and reportedly began using it several weeks ago.
Analysts say it shows Mr Mueller is taking full control of the investigation.
He has already replaced most of the prosecutors originally on the case, in favour of his own legal minds.
The switch from Virginia to Washington is also more practical. Mr Mueller's office will be closer and he knows the Washington federal courthouse better.
What will it look into?
As grand juries are secret, there will be no disclosure. Mr Mueller himself has given no details.
It is clear Mr Mueller is investigating a meeting in June last year, in which Mr Trump's son, Donald Jr, and his son-in-law, Jared Kushner, met a Russian lawyer. Mr Trump Jr has admitted he was promised damaging material on Mr Trump's rival in the presidential race, Democratic candidate Hillary Clinton, though he says none was forthcoming.
According to Reuters news agency, the special counsel is examining whether anyone affiliated with the Trump campaign encouraged the Russians to start releasing material about the Clinton campaign.
Mr Mueller is reportedly seeking to determine whether Mr Trump knew of his son's meeting before it happened, or if he was briefed on it afterwards.
US media have also quoted sources close to the investigation as saying that the special counsel is looking into whether President Trump obstructed justice when he fired FBI chief James Comey, who was leading the previous investigation into Russian interference.
Mr Mueller was then appointed in May by the deputy attorney general of the Department of Justice to continue the inquiry.
Mr Mueller is also reportedly investigating the financial dealings of former Trump campaign chief Paul Manafort as well as those of Michael Flynn, who Mr Trump sacked as national security adviser because he failed to disclose his Russian contacts.
What has the president and his team said?
Mr Trump was his defiant self 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.
The crowd went wild as he continued: "Most people know there were no Russians in our campaign. There never were. We didn't win because of Russia, we won because of you."
Ty Cobb, a lawyer appointed last month as White House special counsel, said he had been unaware a grand jury was being used.
But he added: "Grand jury matters are typically secret. The White House favours anything that accelerates the conclusion of [Mr Mueller's] work fairly. The White House is committed to fully co-operating with Mr Mueller."
Another Trump lawyer, John Dowd, told Associated Press: "With respect to the news of the federal grand jury, I have no reason to believe that the president is under investigation."
On the trail of something big?
Anthony Zurcher, BBC North America reporter
Robert Mueller's special counsel investigation has always been a concern for the Trump administration. Now it's deadly serious business.
With the news that a grand jury has been convened in Washington DC, and that it is looking into the June 2016 meeting between Donald Trump Jr and Russian nationals, it's clear the investigation is focusing on the president's inner circle.
This news shouldn't come as a huge shock, given that Mr Mueller has been staffing up with veteran criminal prosecutors and investigators. It is, however, a necessary step that could eventually lead to criminal indictments. At the very least it's a sign that Mr Mueller could be on the trail of something big - expanding the scope beyond former National Security Adviser Michael Flynn and his questionable lobbying. It also indicates his investigation is not going to go away anytime soon.