Special Counsel Robert Mueller has impaneled a new grand jury in Washington to probe Russia’s interference in the 2016 elections and possible collusion with the Trump campaign, a new report said Thursday.

The development is sign that the investigation is gaining steam, The Wall Street Journal reported, citing a pair of sources close to the probe.

The grand jury began its work in recent weeks, and was considered to be evidence that Mueller’s inquiry isn’t going away any time soon.

A spokesman for Mueller, Joshua Stueve, declined to comment to the paper.

Team Trump has repeatedly attacked the probe as “fake news” and a witch hunt,” and the Kremlin has also denied that it interfered in the race to aid Trump and hurt Hillary Clinton.

But the US intelligence community said there is plenty of evidence of meddling, which prompted an FBI probe, which Mueller was assigned to take over after Trump fired FBI chief James Comey.

“This is yet a further sign that there is a long-term, large-scale series of prosecutions being contemplated and being pursued by the special counsel,” Stephen I. Vladeck, a law professor at the University of Texas, told the paper.

“If there was already a grand jury in Alexandria looking at Flynn, there would be no need to reinvent the wheel for the same guy. This suggests that the investigation is bigger and wider than Flynn, perhaps substantially so.”

Thomas Zeno, a lawyer at the Squire Patton Boggs law firm and a former federal prosecutor, told The Journal that the grand jury was “confirmation that this is a very vigorous investigation going on.”

“This doesn’t mean he is going to bring charges,” Zeno added.

“But it shows he is very serious. He wouldn’t do this if it were winding down.”

Ty Cobb, Trump’s special counsel, told The Journal he wasn’t aware that a new grand jury had been impaneled.

“Grand jury matters are typically secret,” Cobb said.

“The White House favors anything that accelerates the conclusion of his work fairly. The White House is committed to fully cooperating with Mr. Mueller.”

Federal prosecutors had already been using at least one other grand jury, located in Alexandria, Va., as part of their investigation of Michael Flynn, a Trump campaign supporter and his ousted national security adviser.

That probe, which has also been taken over by Mueller’s prosecutors, was probing Flynn’s work as a private citizen for foreign interests.

The revelation came as lawmakers introduced bipartisan Senate bills that aimed to protect Mueller should Trump follow through on his desire to fire the special counsel.

Both bills would require judicial review of any move to oust him, Politico reported.

One of the measures would let Mueller challenge his firing by appealing to a panel of three federal judges — but only after he’d gotten the boot.

The second would force Justice Department officials who were ordered to fire Mueller to go before the judicial panel before he’s canned to explain their reasons.

It is unclear how long the investigation will last, because there is no deadline to wrap it up.