Special counsel Robert Mueller has impaneled a grand jury as the federal investigation into Russia's interference in the presidential election intensifies, according to news reports.

The Wall Street Journal, which broke the story Thursday, reported that the grand jury was based in Washington and began working in recent weeks.

A grand jury has issued subpoenas relating to the controversial June 2016 meeting between the president's son, Donald Trump Jr., and a Russian lawyer, Reuters reported.

Last month, Trump Jr. released emails showing he arranged the meeting in the hopes of obtaining damaging information about his father's election opponent Hillary Clinton – even after he was told it would come from the Russian government. Trump son-in-law and adviser Jared Kushner, along with then-campaign chairman Paul Manafort, were also in the meeting with Russian lawyer Natalia Veselnitskaya and others.

Josh Stueve, a spokesman for Mueller, declined to comment on the report.

President Trump has denounced Mueller's investigation into possible collusion between his associates and Russians who sought to influence the election by hacking Democrats. He's called the investigation a "witch hunt."

The news appeared to take the Trump team by surprise.

Ty Cobb, special counsel to the president, did not know that Mueller had called a 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."

John Dowd, Trump's lead outside lawyer, also said Thursday that he was not aware of Mueller's reported move to impanel a new grand jury in D.C., but said he was confident that the president was not a target of prosecutors.

"With respect to reports of a federal grand jury, I have no reason to believe that President Trump is being investigated," Dowd said.

Democrats in Congress welcomed the development.

"Americans deserve answers. I hope this brings us one step closer," tweeted Sen. Patrick Leahy, D-Vt.

Top Democrat on the House Intelligence Committee, which is also investigating Russia's election meddling, said news that Mueller has impaneled a grand jury "suggests his work is proceeding."

"All the more impt (important) that Congress protect his independence," tweeted Rep. Adam Schiff, D-Calif.