WASHINGTON — Special Counsel Robert Mueller is using a grand jury in Washington as part of an investigation into potential coordination between the Trump campaign and Russia, a person familiar with the probe says.

The use of a grand jury, a standard prosecution tool in criminal investigations, suggests that Mueller and his team of investigators are likely to hear from witnesses and demand documents in the coming weeks and months.

The person who confirmed to The Associated Press that Mueller had turned to a grand jury was not authorized to discuss the investigation by name and spoke on condition of anonymity.

The Wall Street Journal first reported the use of a grand jury.

Grand juries are common vehicles to subpoena witnesses and records and to present evidence, though they do not suggest any criminal charges are near or will necessarily be sought. It was not immediately clear how or whether the Washington grand jury was connected to the work of a separate one in Alexandria, Virginia. That panel has been used to gather information on Michael Flynn, Trump’s former national security adviser.

A spokesman for Mueller’s team did not return an email seeking comment.

“We won. Move on!” Trump demanded in a tweet posted early Friday, a day after he leveled a host of broadsides against Democrats and investigators at a rally in Huntington, West Virginia.

“We know that the nature of these investigations become fishing expeditions where you’re just throwing Jello up against the wall and hoping something will stick,” said White House senior counselor Kellyanne Conway on “Fox and Friends.”

Mueller’s reliance on a grand jury is the “logical next step in this investigation” given that it’s the traditional method for prosecutors to gather evidence, said Washington defense lawyer Jacob Frenkel.

“The use of the grand jury neither escalates, nor establishes a timeline for, the investigation,” he added.

President Donald Trump, during an appearance Thursday evening at a rally in Huntington, West Virginia, challenged Democrats to either continue their “obsession with a hoax” or begin serving the interests of the American people.

“I just hope the final determination is a truly honest one,” he said, “which is what the millions of people who gave us our big win in November deserve and what all Americans who want a better future want and deserve.”

Lawyers for Trump said earlier they were unaware of the existence of a grand jury and had no information to suggest the president himself was under federal investigation.

It was not clear what witnesses might appear before the grand jury or what evidence it might be accumulating or presented with.

Though there is “considerable deference to the prosecutors and their recommendations” when it comes to a grand jury, “the mere fact of presenting evidence to the grand jury does not obligate prosecutors in any way to ask them to return an indictment,” Frenkel said.

News of the grand jury came as senators introduced two bipartisan bills aimed at protecting Mueller from being fired by Trump, with both parties signaling resistance to any White House effort to derail the investigation into Russian meddling in last year’s election.

Trump attorney Jay Sekulow told Fox News on Thursday that “the president is not thinking about firing Robert Mueller so the speculation that’s out there is just incorrect.”

He also downplayed the significance of the grand jury, calling it “a standard operating procedure when you’ve got a situation like this.”