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Special Counsel Robert Mueller departs after briefing members of th…

What we know ― and what we don’t ― about Mueller’s grand jury

Robert Mueller, the special counsel overseeing the federal investigation into Russia’s role in the 2016 election and possible ties to President Donald Trump’s campaign, has called on a grand jury, the Wall Street Journal reported — a sign that his probe could be expanding.

The Journal, which broke the news Thursday, said the news indicated a lengthy road ahead for the investigation.

What are grand juries? A grand jury is called when prosecutors want to review evidence — including calling witnesses — to determine whether there’s probable cause to pursue criminal charges; it doesn’t necessarily mean charges will follow. (In a regular jury, jurors are weighing the facts in an individual case that has already come to trial). MSNBC goes into further detail here.

This isn’t all that surprising, says Renato Mariotti, a former federal prosecutor who now defends trading firms and individuals in federal cases in Chicago. A grand jury in the Eastern District of Virginia is reportedly already looking at former National Security Adviser Michael Flynn’s relationship with Russia, Mariotti said.

What it means: “Mueller has made an independent determination that there is reason to believe that a crime has been committed and should be investigated – it’s why you would want to open a grand jury,” Mariotti said.

This is an early step, not a late step in the investigations, Mariotti said: Calling a grand jury is “a prosecutorial tool to issue subpoenas, or require people to testify under oath unless they take the fifth.” In the grand jury that looked at what led to the 2003 public identification of Valerie Plame, a covert CIA officer, prosecutors called more than 20 witnesses, Mariotti said.

READ MORE: Can Congress protect special counsel Mueller from being fired?

Why it matters: Reports late last month suggested the president was considering removing Mueller from the investigation. Trump told the New York Times that Mueller would cross a red line if he began looking at the finances of the president and his family. Bloomberg reported that week that Mueller was “examining a broad range of transactions involving Trump’s businesses as well as those of his associates.”

Trump’s remarks to the Times worried some lawmakers, some of whom introduced legislation on Thursday that would protect the special counsel role and reaffirm its independence from the executive branch.

Trump signed new sanctions against Russia earlier this week, but called the legislation “seriously flawed,” adding that it “makes it harder for the United States to strike good deals for the American people.”

Meanwhile, at the White House: White House Special counsel Ty Cobb told the NewsHour he wasn’t aware of a grand jury, but “… Comey said three times the President is not under investigation and we have no reason to believe that has changed.” He said the White House “is committed to fully cooperating with Mr. Mueller” and “favors anything that accelerates the conclusion of his work fairly.”

What’s next? The next few months will bring grand jury subpoenas and some testimony, Mariotti says. It’s important to note a grand jury “is a necessary step to an indictment … but that doesn’t necessarily tell us whether in this case there’ll be an indictment sought,” Mariotti says.

NewsHour’s Ellis Kim reported for this story.