Special Counsel Robert Mueller convened a grand jury in Washington as part of an investigation into potential coordination between the Trump campaign and Russia, sources familiar with the probe said on Thursday.
The use of a grand jury, a standard prosecution tool in criminal investigations, suggests that Mueller and his team of investigators are likely to hear from witnesses and demand documents in the coming weeks and months.
"It's a significant escalation of the process," national security attorney Bradley Moss said.
Sources who confirmed that Mueller had turned to a grand jury were not authorised to discuss the investigation and spoke on condition of anonymity. They said the grand jury was assembled in recent weeks.
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 US President Donald Trump's favour. Mueller, who was appointed as special counsel in May, is leading the probe, which also examines potential collusion by the Trump campaign with Russia.
"You don't impanel a grand jury unless your investigation has discovered enough evidence that you feel reflects a violation of at least one, if not more, criminal provisions," Moss said.
"If you secure an indictment, your next step is to arrest the defendant."
trump will probably think a grand jury is a compliment, "Did you hear? They had to get a grand jury. And it was so grand. The grandest."— Michelle Wolf (@michelleisawolf) August 4, 2017
Why a grand jury?
A grand jury comprises of a group of ordinary citizens working behind closed doors, considers evidence of potential criminal wrongdoing that a prosecutor is investigating and decides whether charges should be brought.
The grand jury has issued subpoenas (a writ ordering a person to attend a court) in connection with a June 2016 meeting that included Trump's son, his son-in-law and a Russian lawyer, two sources said.
It was not immediately clear to whom subpoenas were issued or what evidence might be presented to the jury and the sources did not elaborate.
Some lawyers said it would put pressure on potential witnesses to cooperate with Mueller's investigation.
"When someone gets a subpoena to testify, that can drive home the seriousness of the investigation," said David Sklansky, a professor at Stanford Law School and a former federal prosecutor.
Mueller impaneling a Grand Jury in Russia Probe and Trump Voters are chanting lock her up in West Virginia right now. IGNORANCE HAS NO CURE.— PROUD RESISTER (@ProudResister) August 3, 2017
A spokesman for Mueller's team did not return an email seeking comment.
Mueller's reliance on a grand jury is the "logical next step in this investigation" given that it's 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.
Trump under federal probe?
Lawyers for President Donald 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 attorney John Dowd said.
News last month of the meeting between Donald Trump Jr and a Russian lawyer whom he was told had damaging information about his father's presidential rival, Democrat Hillary Clinton, fuelled questions about the campaign's dealings with Moscow.
The Republican president has defended his son's behaviour, saying many people would have taken that meeting.
Trump's son-in-law and White House senior adviser Jared Kushner and former Trump campaign manager Paul Manafort also attended the meeting.
One of the sources said major Russian efforts to interfere in the election on Trump's behalf began shortly after the June meeting, making it a focus of Mueller's investigation.
The "you're fired" defence
Mueller was appointed following the firing by 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.
Important. Trump can't "fire" a grand jury. https://t.co/YszNmfXAj3— Adam (@aalali44) August 3, 2017
News of the grand jury came as senators introduced two bipartisan bills aimed at protecting Mueller from being fired by Trump, with both parties signalling resistance to any White House effort to derail the investigation into Russian meddling in last year's election.
Trump attorney Jay Sekulow told Fox News on Thursday that "the president is not thinking about firing Robert Mueller so the speculation that's out there is just incorrect."