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”.
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.
Although they consider whether evidence in any case is strong enough to issue indictments for a criminal trial, their use does not mean such an indictment is imminent or even probable.
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.