Source: Mueller Using D.C. Grand Jury In Russia Probe

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Updated at 8:30 p.m. ET

Special counsel Robert Mueller is using a grand jury in Washington, D.C., in connection with his investigation into Russian efforts to influence the 2016 presidential election and into possible collusion between Russia and top aides to the Trump campaign, a source with knowledge of the investigation confirms to NPR's Peter Overby. The source did not want to be identified because of the sensitivity of the matter.

The Wall Street Journal first reported Thursday that Mueller was using a grand jury. The latest development signals that the former FBI director's investigation is "growing in intensity," with the grand jury beginning work in recent weeks, the Journal reported.

A spokesman for Mueller declined to comment to NPR.

Ty Cobb, a recently appointed White House special counsel focused on the Russia probes, said in a statement provided by White House press secretary Sarah Huckabee Sanders on Thursday that he wasn't aware Mueller had started using a grand jury.

"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."

President Trump has maintained there was no collusion between Russia and his campaign during the 2016 election. And Trump has, at times, cast doubt on the determination of the U.S. intelligence community that Russia was behind various efforts to interfere in the election, including the hacking of emails belonging to individuals and organizations aligned with the Democrats and the strategic release of those emails at key points in the campaign last year.

But in a statement Wednesday announcing that he had signed into law a bill imposing sanctions on Russia, Trump said he supported "making clear that America will not tolerate interference in our democratic process and that we will side with our allies and friends against Russian subversion and destabilization."

Trump has repeatedly called Mueller's probe a "witch hunt," and some media reports have indicated the president has looked into ways to undercut or even fire Mueller.

Speaking at a campaign rally on Thursday in West Virginia, Trump didn't directly address the reports about Mueller and the grand jury, but he continued to blast the Russia investigation as a "total fabrication" that was "just an excuse for the greatest loss in the history of American politics."

"The reason why Democrats only talk about the totally made-up Russia story is because they have no message, no agenda and no vision," the president said to cheers.

"Most people know there were no Russians in our campaign. There never were," Trump continued. "We didn't win because of Russia, we won because of you."

"What the prosecutors should be looking at is Hillary Clinton's 33, 000 deleted emails," Trump declared, reviving a favorite campaign trail line that, in turn, elicited a chant of "Lock her up! Lock her up!" from the crowd that was another hallmark of his raucous rallies.

"With respect to the impaneling of the grand jury, we have no reason to believe that the president is under investigation," Jay Sekulow, a lawyer on the president's outside legal team, told NPR's Tamara Keith earlier on Thursday.

The revelation that Mueller's investigation is utilizing a grand jury comes as CNN reports that investigators have zeroed in on "Trump and his associates' financial ties to Russia as one of the most fertile avenues for moving their probe forward," with sources saying that the "web of financial ties could offer a more concrete path toward potential prosecution than the broader and murkier questions of collusion in the 2016 campaign." That includes ongoing investigations into former White House national security adviser Michael Flynn and Trump's former campaign chairman, Paul Manafort.

But Mueller dipping into Trump's own personal finances is exactly what the president argued to the The New York Times last month would be a "violation" of the scope of investigation delineated at the time Mueller was appointed by the Justice Department.

Reuters is also reporting that grand jury subpoenas have been issued regarding Donald Trump Jr.'s June 2016 meeting with a Russian attorney who the president's son was told would have incriminating information about Democratic presidential nominee Hillary Clinton.