Grand jury summons have been issued in connection with a June 2016 meeting that included President Donald Trump's son, his son-in-law and a Russian lawyer, two sources told Reuters on Friday (NZT), in a sign that special counsel Robert Mueller's investigation is gathering pace.
The sources added that Mr Mueller had convened a grand jury in Washington to help investigate allegations of Russian meddling in the 2016 US election.
Russia has loomed large over the first six months of the Trump presidency.
US intelligence agencies have concluded that Russia worked to tilt the presidential election in Mr Trump's favor.
Mr Mueller, who was appointed special counsel in May, is leading the probe, which also examines potential collusion by the Trump campaign with Russia.
Moscow denies any meddling and Mr Trump denies any collusion by his campaign, while regularly denouncing the investigations as political witch hunts.
Mr Mueller's use of a grand jury could give him expansive tools to pursue evidence, including issuing subpoenas and compelling witnesses to testify.
A spokesman for Mr Mueller declined to comment.
A grand jury is a group of ordinary citizens who, working behind closed doors, considers evidence of potential criminal wrongdoing that a prosecutor is investigating and decides whether charges should be brought.
"This is a serious development in the Mueller investigation," said Paul Callan, a former prosecutor.
"Given that Mueller inherited an investigation that began months ago, it would suggest that he has uncovered information pointing in the direction of criminal charges. But against whom is the real question."
US stocks and the dollar weakened following the news, while US Treasury securities gained.
Mr Trump is holding a rally in West Virginia on Friday afternoon (NZ time). He has not yet tweeted about the latest developments in the Russia investigation.