In a federal courthouse in Washington, D.C., between 16 and 23 citizens have reportedly been meeting in secret for weeks, tasked with deciding whether to bring charges in a probe of alleged collusion between U.S. President Donald Trump's campaign and Russia.

Collectively, they're known as a federal grand jury. And on Thursday, reporters learned they were "impanelled" — or brought together — by the all-star team of lawyers led by special counsel Robert Mueller.

What does this mean?

Activating the grand jury is a sign Mueller's Trump-Russia investigation is heating up. Equipped with this tool, Mueller can request the issuing of subpoenas for documents and testimonies.

It marks the next phase in his investigation — "the most critical phase," according to attorney Seth Abramson, an assistant professor at the University of New Hampshire.

"That's because it's the phase in which he doesn't have to rely on voluntary production of documents or voluntary testimony from important witnesses."

Any requested documents — which could include phone records, text messages, hotel and restaurant receipts and emails — get shared with the grand jury for consideration as evidence against a target of the investigation. 

Subpoenaed witnesses must appear in front of the grand jury to be questioned by first Mueller's team then the jurors themselves. Witnesses can plead the Fifth Amendment to invoke their right against self-incrimination.

How does a grand jury work?

The grand jury is a mechanism to get an indictment, or criminal charge. The main reason to impanel a grand jury is that federal prosecutors believe they have enough evidence of a crime, or crimes, committed.

"This is a signal that they've found something of interest," says former federal prosecutor Barak Cohen.

"That doesn't mean an indictment is likely, or that a crime probably happened," he says. "It just means that Mueller and his team have enough evidence now to believe they should cross the threshold."

If 12 of the members of the grand jury agree that the evidence before them meets the threshold of "probable cause" of a crime, an indictment may be returned.

Trump Vacation

Marine One carries Trump from the White House on Friday, August 4, as he starts a 17-day vacation. (J. David Ake/Associated Press)

Should we be surprised by this?

Not really. Legal experts saw this coming.

For that reason, Jim Trusty, a former chief of the organized crime and gang section of the Department of Justice, doesn't see this as a dramatic move.

"Frankly, I'd be surprised if they hadn't opened one," says Trusty, now a partner at Ifrah Law. "For any investigation that has even just the raw potential to lead to criminal charges, you'd expect this."

Douglas McNabb, a federal criminal defence attorney who has handled cases for 35 years, says at this point the opening of a grand jury is a "rubber stamp" process.

Does this mean indictments are coming?

As the saying goes: "A federal grand jury could indict a ham sandwich."

Abramson says, "It's unthinkable there would be no indictment," though who would receive indictments, and on which charges, is much harder to say.

According to a Washington Post analysis from 2014, out of about 163,000 cases in 2009-2010, grand juries declined to indict in just 11 cases. That means 99.99 per cent of grand juries returned an indictment.

Those are compelling numbers, McNabb says.

'It's unthinkable there would be no indictment.' - Seth Abramson, University of New Hampshire

"And Mueller's got 16 top-notch federal prosecutors on this thing."

However, legal experts are careful to remind people not to jump to conclusions.

It's important to note that federal grand juries are entirely one-sided presentations of evidence. No defence attorneys attend and jurors aren't shown documents to argue against the government's case.

As a burden of proof, "probable cause" is considered a relatively low bar to clear, requiring only a reasonable basis to believe a crime occurred. By comparison, the "preponderance of evidence" level of proof means the evidence shows a crime was more than likely.

Wasn't there already a grand jury on this?

Federal prosecutors have been using a grand jury based in Alexandria, Va., which was looking into Trump's former national security adviser Michael Flynn. CNN reported in the spring that prosecutors had issued subpoenas to Flynn's associates for records related to his business dealings with Russia.

That Mueller has now tapped a separate grand jury in Washington is notable because it indicates an expansion.

"This new grand jury situated in D.C. is telling," McNabb says. "I think it says there's much, much more than just the Flynn matter."

Abramson notes there's another big implication to impanelling a grand jury in D.C.: "It's clear that this is the jurisdiction in which you would bring charges against the president of the United States," he says, as well as former Trump campaign manager Paul Manafort, Trump's son-in-law Jared Kushner and the president's eldest son, Donald Trump Jr.

kushner trump jr manafort

Jared Kushner, left, Donald Trump Jr., centre, and Paul Manafort were all called to testify before Senate committees last month regarding the 2016 election. (Carolyn Kaster/Matt York, Matt Rourke/AP)

Grand juries are secret. How did this leak?

Judging by the flurry of recent news scoops about tumult in Trump's White House, the short answer is that Washington doesn't appear to be great at keeping secrets these days.

"None of this should even be out there," says Trusty. "It's reflective of an incredibly leaky ship all around." 

Grand juries are secret proceedings. Jurors and prosecutors are bound to secrecy, but witnesses are not.

"A witness can walk out and say, 'Man, they're out to get me,' and talk about it," Trusty says. "But generally speaking, most witnesses aren't going to want to go out and celebrate what's happening."

What does this mean for Mueller and Trump?

Impanelling a grand jury gives Mueller some political cover against the prospect of Trump trying to get him fired.

"Once it formally becomes a grand jury investigation, it becomes a little more significant, a little more formal," Cohen says.

"It's a lot clearer if somebody's trying to interfere."

US Russia Sanctions

Trump meets with Russian President Vladimir Putin at the G-20 Summit in Hamburg, July 7, 2017. This week, Trump reluctantly signed what he called a 'seriously flawed' bill imposing new sanctions on Russia. (Evan Vucci/Associated Press)

The president's special counsel, Ty Cobb, says the president is not under investigation, and it remains up for debate whether a sitting president can even be indicted. Constitutional scholars argue indictment is possible, but a president can't be tried for those offences until he or she leaves office or is impeached.

As for the "indicting a ham sandwich" analogy, Trusty feels that's overplayed. He cautions against people getting ahead of themselves.

"Literally what we're doing right now is a routine check that we have bread, ham, mustard and cheese — just the basic building blocks of an investigation," he says. "Don't read it as doomsday, or X months or weeks before an indictment of the following people."

Grand juries can last between 12 and 18 months, or even years, depending on the complexity of the case.