The Justice Department's special counsel Robert Mueller has empaneled a grand jury in the nation's capital to hear evidence that Russia interfered in the 2016 U.S. elections, and to investigate whether President Donald Trump's campaign colluded with Moscow to tilt the White House race in his direction.

And the panel has already issued its first subpoenas, according to a Reuters report, making demands related to a 2016 Trump Tower meeting that included Donald Trump Jr. and a Russian lawyer who promised to disclose – but never delivered – dirt on Hillary Clinton.

Grand juries can allow prosecutors to subpoena documents, interview witnesses under oath, and obtain criminal indictments where the evidence warrants it.

Going that route signals that the probe is entering a new phase, and that Mueller believes he will need to demand documents and sworn testimony from a number of people.

Prosecutors are also probing Trump and Trump family members financial ties, CNN reported.

The Wall Street Journal reported the existence of the grand jury, which has been in session for weeks under the guidance of Mueller, himself a former director of the FBI.

Robert Mueller, the special counsel investigating so-far unsubstantiated charges that Donald Trump's presidential campaign colluded with Russia to tilt the 2016 election, has empaneled a grand jury to hear testimony and examine evidence

President Donald Trump has denied repeatedly that he or anyone in his presidential campaign colluded with Russia to undermine or otherwise impact the election

Reuters reports that the first grand jury subpoenas have already begun to fly, related to a 2016 Trump Tower meeting that included Donald Trump Jr. (left) and a Russian lawyer

The subpoenas indicate that Mueller will use his investigative powers to scrutinize a meeting between the president's eldest son and a Kremlin-linked lawyer in June of 2016.

Donald Trump Jr. set up the meeting after being contacted via email by music publicist Robert Goldstone, who promised dirt on Clinton and mentioned Russian government support for Donald Trump.

Trump Jr. immediately agreed to the meeting and subsequently invited Trump son-in-law Jared Kushner and former Trump campaign chair Paul Manafort to join him.

The meeting was attended by Russian lawyer Natalia Veselnitskaya, a leading proponent of an effort to roll back anti-Russia sanctions in the 2012 Magnitsky Act – a U.S. law deplored by Russian President Vladimir Putin.

When the outlines of the meeting first came to light, Trump Jr. initially stated that it was mainly about Russia's prohibition on Americans adopting children from there, a ban that came about in response to U.S. sanctions.

President Trump 'weighed in' on that statement while it as being written, the White House later confirmed.

Trump Jr. ultimately released the email chain revealing the Clinton connection.

Russian lawyer Natalia Veselnitskaya attended the 2016 meeting in Trump Tower which also included Donald Trump Jr., Jared Kushner and Paul Manafort

The president has forcefully denied that he and his campaign were involved with the Kremlin's alleged efforts, calling the investigation a 'witch hunt.'

But Washington, D.C. is an exceedingly Democratic district where Trump won just 4 per cent of the vote, meaning few potential grand jurors will be sympathetic to him.

Three U.S. intelligence agencies concluded late last year that Russia was indeed behind an effort to undermine America's presidential election.

Ty Cobb, special counsel to the president, said in a statement that he didn't know if a grand jury had been empaneled.

'Grand jury matters are typically secret,' he said.

But Cobb suggested that the administration isn't spooked by the new development.

'The White House favors anything that accelerates the conclusion of his work fairly,' he said, adding that 'the White House is committed to fully cooperating with Mr. Mueller.'

The White House press office added that former FBI Director Jim Comey 'said three times the President is not under investigation and we have no reason to believe that has changed.'

Mueller (left) has a close relationship with ousted FBI director James Comey (right), a fact that Trump has used to try to delegitimize the probe as biased and a 'witch hunt'

Thomas Zeno, a 29-year federal prosecutor before he entered private legal practice, told the Journal that Mueller's move 'doesn't mean he is going to bring charges' against anyone.

'But it shows he is very serious. He wouldn't do this if [the investigation] were winding down.'

Special counsel investigations can proceed for years, and there has been no indication from Mueller or his team about how long the Russia probe could take to unfold.

Federal grand juries typically sit for 18 months, and can hear evidence on more than one case. Judges can extend that period after the 18-month period ends.

Federal grand jurors are expected to be available every weekday during their term of service, although the grand jury may not be called on to convene every day.

Its role is to hear evidence from witnesses and examine documents with guidance from prosecutors, in order to decide whether 'probable cause' exists to charge someone with a crime.

Mueller's grand jury, unlike most, will be tasked with examining classified information – a feature that will add complexities to the situation.

Trump has sought to publicly undermine the credibility of Mueller's legal team, telling a Fox news Channel interviewer last month that they are largely Democratic partisans engaged in a fishing expedition to harm his presidency.

'I can say that the people that have been hired are all Hillary Clinton supporters, some of them worked for Hillary Clinton,' he said.

In addition, Trump has pointed out repeatedly that Mueller has a close relationship with James Comey, a former FBI director whom the president fired this year.

The president has made the unusual decision to go after Mueller in public – and told the New York Times he would consider it to be crossing a 'red line' if Mueller were to probe his finances or his family finances.

Mueller's decision to impanel a new grand jury added new urgency to the question of whether Trump will ultimately fire him.

The president avoided a question about whether he would fire Mueller as he approached Marine One Thursday en route to a campaign event in West Virginia.

Two bipartisan pairs of senators have introduced legislation that would allow the Mueller to stay on if Trump fired him. The special counsel could seek a review in court if he got fired, according to the bill introduced Thursday by GOP Sen. Thom Tillis of North Carolina and Democratic Chris Coons of Delaware.

Only a Senate-confirmed attorney general would be able to carry out his firing, under the bill – a provision in response to Trump's public attacks on Attorney Gen. Jeff Sessions, who recused himself from the Russia probe, infuriating Trump.

Trump outside attorney Jay Sekulow told Fox News: 'Well the president is not thinking about firing Robert Mueller so the speculation that’s out there is just incorrect.'