But what exactly does a federal grand jury do, and what does it mean that Mueller has started utilizing one in the nation's capital?
Here's what you need to know.
How is a grand jury different from a common jury?
Grand juries operate in secret, and prosecutors present their case by laying out evidence to support it, including by using in-person witness testimony.
Comprised of between 16 and 23 members of the public, grand juries usually last for 18 months, although that period can be extended under certain circumstances.
In order to indict a defendant on any charges, prosecutors must convince the grand jury that there is “probable cause” to believe a crime was committed.
What kind of authority does a grand jury have?
To help determine whether such “probable cause” exists, grand juries have special authority to take their own investigative steps, which are often guided by prosecutors.
Federal authorities can use grand juries as a tool in their investigations and use their authority to issue subpoenas to demand that uncooperative suspects, witnesses, and companies hand over private documents or testify behind closed doors.
It’s standard-operating procedure for a single grand jury to handle multiple matters over its lifespan. They may indict an alleged drug trafficker on a Monday, and then approve a subpoena for evidence in another case on Wednesday.
In some cases, prosecutors will use a grand jury for the sole purpose of collecting evidence in an investigation even when it’s not yet clear that a crime was likely committed.
Where does the Russia probe fit in?
Earlier this year, federal prosecutors were using a federal grand jury in Alexandria, Virginia, to collect evidence in the federal probe of Trump associates and any potential ties they could have to Russian operatives.
But in mid-May, after Mueller became special counsel and started putting together his team, the entire federal probe became based out of an office building in downtown Washington, D.C.
Then, weeks ago, Mueller’s prosecutors began using a federal grand jury that sits in the federal courthouse in Washington, just a few blocks away from their offices.
In certain circumstances, federal authorities might seek to impanel a special grand jury to focus on one specific matter.
There is no indication, however, that Mueller has been using a special grand jury for the Russia probe.
There is also no indication that the grand jury his team is now using in Washington was specifically impaneled for the Russia investigation.