The FBI s investigation into potential collusion between President Donald Trump s campaign and Russia appeared to kick into higher gear with news this week that special counsel Robert Mueller had enlisted a federal grand jury in Washington as part of his efforts. The grand jury reportedly began working in recent weeks, issuing subpoenas for documents related to former national security adviser Michael Flynns business dealings.

The high-profile and intensely political nature of this investigation could put this grand jury under unusual pressure.

The panel is reportedly not dedicated to the Russia probe alone and could have other cases on its docket as it meets weekly or every other week until the completion of its term. Grand jury proceedings take place largely in secret, meaning the public will likely learn little about what the jurors are hearing.

But legal experts say the use of the grand jury suggests Muellers investigation is diving in deeper.

The duty of a grand jury is to decide if there is probable cause to pursue criminal charges. Grand juries have the power to issue subpoenas for documents and for witness testimony, which is delivered under oath without the presence of a defense attorney. Prosecutors can thus use grand juries to investigate more expansive matters. If or when the prosecutors think theres cause for an indictment, they will ask the grand jury to vote.

A federal grand jury doesnt need to reach a unanimous decision. Of the 16 to 23 Washington residents on the panel looking into possible collusion with Russia, only 12 need to vote yes to indict someone.

Grand juries differ from trial juries, or petit juries, in a few other substantive ways. Although courts do vet grand jurors, the individuals who sit on these panels dont go through the extensive process of voir dire, in which lawyers and judges ask about their opinions and backgrounds to ensure they can serve impartially on specific cases.

Grand juries line em up, file em in, sit em down and off you go, because grand juries often hear all manner of cases, said Andrew Leipold, a professor at the University of Illinois College of Law and an expert in grand juries. They might hear several cases in a day, a drug crime here, a gun possession there, a fraud case, and so its impossible really to sort of track the way you would with a trial jury what are your particular biases and what are your leanings about this and that.

In Washington, which voted almost 91 percent for Hillary Clinton in November, the partisan makeup of the grand jury likely leans heavily against Trump.

The D.C. grand jury is one of at least two such federal panels examining the Trump team in the wake of the 2016 election. A grand jury in eastern Virginia, not far from Washington, has been looking into Flynns ties to foreign agents. That region of Virginia also leans heavily blue.

Additional grand jury subpoenas have reportedly been issued in connection with the June 2016 meeting between top Trump campaign officials including the presidents oldest son, Donald Trump Jr. ; son-in-law Jared Kushner ; and then-campaign manager Paul Manafort and a Russian lawyer with links to the Kremlin.