Sam Nunberg on Friday became the first recognizable Trump campaign aide to appear before a federal grand jury in relation to special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's investigation into Russian meddling in the 2016 election.

His testimony came days after Nunberg brazenly vowed in multiple media interviews he would defy a grand jury subpoena from the special counsel. He ultimately reversed course and agreed to appear.

In an interview with ABC News aired Saturday, Nunberg said he answered "a lot of questions" during six hours of testimony and was "grilled" about other members of the campaign. As a cooperating witness, he may be required to testify again.

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The grand juries involved in Mueller's investigation have been hearing evidence and witness interviews since long before Nunberg's appearance this week, and have so far approved a number of indictments.

Here are five things to know about the grand jury:

A grand jury has been involved in the Russia probe since at least August

News that Mueller had tapped grand juries in Washington, D.C., and Virginia emerged in August, signaling that the special counsel's investigation had moved into an aggressive new phase.

The use of grand juries is typical for complex probes, because it allows investigators to seek subpoenas and indictments, and to interview witnesses in a more formal setting.

Juries meet relatively often on a wide variety of topics

The D.C. grand jury appears to meet at least once a week or more, according to a CNN report.

Federal grand juries are made up of anywhere between 16 and 23 people, who are sworn to secrecy on matters regarding the proceedings. The grand jury then hears evidence from prosecutors to decide whether or not criminal charges should be brought.

At least 12 jurors must agree that charges are warranted in order to approve an indictment.

The grand jury has heard testimony from at least one Trump campaign aide

Nunberg was the first known Trump campaign aide to appear before a grand jury in relation to Mueller's investigation.

But the grand jury has already heard testimonies from a number of associates of former Trump campaign figures.

Jason Maloni, a former spokesman for ex-Trump campaign chairman Paul Manafort Paul John ManafortOur Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam Conspicuous by their absence from the Republican Convention MORE, appeared before a grand jury in September. And Bijan Kian, an Iranian-American businessman and associate of former national security adviser Michael Flynn, testified before a grand jury in November.

There have so far been 22 indictments and plea deals

Manafort and his longtime associate Richard Gates were the first indicted in Mueller's investigation. Gates entered a plea deal last month. Manafort will reportedly face two trials on 18 criminal charges.

But in the weeks that followed those indictments, George PapadopoulosGeorge Demetrios PapadopoulosTale of two FBI cases: Clinton got warned, Trump got investigated Trump says he would consider pardons for those implicated in Mueller investigation New FBI document confirms the Trump campaign was investigated without justification MORE, a former Trump campaign adviser, and Flynn pleaded guilty to lying to investigators about their contacts with Russians.

An indictment unsealed last month also named 13 Russian nationals and three Russian companies that prosecutors accused of meddling in the 2016 presidential election.

On top of that, Richard Pinedo, a California man, pleaded guilty last month to an identity theft charge stemming from Mueller's broader investigation into Russia's role in the 2016 election.

And Alex van der Zwaan, a lawyer and the son-in-law of a Russia-based billionaire, pleaded guilty last month to lying to investigators about his contacts with a former Trump campaign aide.

The grand jury shows no signs of wrapping up

So far, there are no public signs that the grand jury's term is coming to a close.

Courts can extend a grand jury's period of service for as long as 18 months, though during that period, the grand jury can only conduct investigative work started in its original term.

What's more, Nunberg's assertion to ABC News that there is "a lot" to Mueller's investigation could suggest that the grand jury may be around for a while.