Special Counsel Robert Mueller has impaneled a grand jury in Washington to investigate Russia’s interference in the 2016 elections, a sign that his inquiry is growing in intensity and entering a new phase, according to people familiar with the matter.

The grand jury, which began its work in recent weeks, signals that Mr. Mueller’s inquiry will likely continue for months. Mr. Mueller is investigating Russia’s efforts to influence the 2016 election and whether President Donald Trump’s campaign or associates colluded with the Kremlin as part of that effort.

A spokesman for Mr. Mueller, Joshua Stueve, declined to comment. Moscow has denied seeking to influence the election, and Mr. Trump has vigorously disputed allegations of collusion. The president has called Mr. Mueller’s inquiry a “witch hunt.”

Ty Cobb, special counsel to the president, said he wasn’t aware that Mr. Mueller had started using a new grand jury. “Grand jury matters are typically secret,” Mr. 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.”

Before Mr. Mueller was tapped in May to be special counsel, federal prosecutors had been using at least one other grand jury, located in Alexandria, Va., to assist in their criminal investigation of Michael Flynn, a former national security adviser. That probe, which has been taken over by Mr. Mueller’s team, focuses on Mr. Flynn’s work in the private sector on behalf of foreign interests.


Mr. Mueller requested in recent weeks that Chief Judge Beryl A. Howell of the U.S. District Court for the District of Columbia impanel a grand jury for his use. She granted the request, according to people familiar with the probe.

Grand juries are investigative tools that allow prosecutors to subpoena documents, put witnesses under oath and seek indictments, if there is evidence of a crime. Legal experts said Mr. Mueller’s decision suggests he believes he will need to subpoena records and take testimony from witnesses.

A grand jury in Washington is also more convenient for Mr. Mueller and his 16 attorneys—they work just a few blocks from the U.S. federal courthouse where grand juries meet—than one that is 10 traffic-clogged miles away in Virginia.

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“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,” said Stephen I. Vladeck, a law professor at the University of Texas. “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 federal prosecutor for 29 years before becoming a lawyer at the Squire Patton Boggs law firm, said the grand jury was “confirmation that this is a very vigorous investigation going on.”

“This doesn’t mean he is going to bring charges,” Mr. Zeno cautioned. “But it shows he is very serious. He wouldn’t do this if it were winding down.”

Another sign the investigation is ramping up: Greg Andres, a top partner in a powerhouse New York law firm, Davis Polk & Wardwell LLP, has joined Mr. Mueller’s team.

Mr. Andres, a former top Justice Department official who also oversaw the criminal division of the U.S. attorney’s office in Brooklyn, wouldn’t leave his private-sector job for a low-level investigation, Mr. Zeno said. “People like Greg Andres don’t leave private practice willy-nilly “The fact he is being added after a couple of months shows how serious this is and that it could last a long time,” Mr. Zeno said.


Mr. Andres couldn’t be reached for comment.

The developments unfolded amid a new sign of concern by Congress that Mr. Mueller’s independence needs to be protected. Sens. Thom Tillis (R., N.C.) and Chris Coons (D., Del.) introduced legislation Thursday making it harder for Mr. Trump to fire Mr. Mueller. Under the legislation, a special counsel could challenge his or her removal, with a three-judge panel ruling within 14 days on whether the firing was justified.

If the panel found no good cause for the firing, the special counsel would immediately be reinstated.

As U.S. sanctions against Russia for its interference in the 2016 presidential election move forward, here's a look at various contacts between President Trump's associates and Russians. Photo: Getty

The bill follows a similar effort from Sens. Lindsey Graham (R., S.C.) and Cory Booker (D., N.J.) “The introduction of two bills with two different bipartisan pairs strengthens the message that there is broad concern about this,” said Mr. Coons, who said Mr. Tillis approached him on the Senate floor about teaming up on legislation.


According to a January report from the U.S. intelligence community, the highest levels of the Russian government were involved in directing the electoral interference. Its tactics included hacking state election systems; infiltrating and leaking information from party committees and political strategists; and disseminating through social media and other outlets negative stories about Democratic nominee Hillary Clinton and positive ones about Mr. Trump, the report said.

It is unclear how long Mr. Mueller’s investigation will last, and there is no deadline for its completion. The probe is complicated by the classified nature of much of the information Mr. Mueller’s team is reviewing. Evidence of its sensitivity came in June when Mr. Mueller moved from his temporary offices to a nearby secure facility that his representatives have declined to identify.

While working closely with Federal Bureau of Investigation agents, Mr. Mueller has assembled a team of accomplished prosecutors and lawyers specializing in criminal and national security law.

Twelve attorneys are on temporary assignment to the special counsel’s office from the Justice Department or FBI, and three came from Mr. Mueller’s firm of WilmerHale. Mr. Andres is the most recent addition.

Mr. Trump has questioned the neutrality of Mr. Mueller’s office, telling Fox News he is concerned that Mr. Mueller’s prosecutors are “Hillary Clinton supporters” and that Messrs. Mueller and former Federal Bureau of Investigation Director James Comey are friends. Mr. Comey was a top Justice official in the George W. Bush administration when Mr. Mueller was the FBI director; both were appointed by Republicans. Mr. Trump fired Mr. Comey from the FBI position in May.

Those who know both men said they aren’t social friends, though they respect each other and had a solid relationship in government.

At least eight members of Mr. Mueller’s team have given to Democratic candidates, including the presidential campaigns of Barack Obama and Mrs. Clinton, according to Federal Election Commission records. At least one—James Quarles, a member of the Watergate Special Prosecution Force—has donated to those in both parties. Mr. Andres in March supported a Democratic lawmaker, donating $2,700 to Kirsten Gillibrand (D., N.Y.), according federal campaign disclosure records.

Mr. Mueller made two contributions in 1996 to Republican William Weld, then a candidate for a U.S. Senate seat in Massachusetts, according to the Center for Responsive Politics, which tracks money in politics.

—Siobhan Hughes and Rebecca Ballhaus contributed to this article.

Write to Del Quentin Wilber at del.wilber@wsj.com and Byron Tau at byron.tau@wsj.com