Robert Mueller, the special counsel in the Russia probe, has impaneled a grand jury to look into Russia's involvement in the 2016 election, according to a report.

The Wall Street Journal reported Thursday that the grand jury has begun its work in Washington in recent weeks.

News that Mueller has impaneled a grand jury signals that the Justice Department's investigation into Russian meddling and ties between the Trump campaign and Russian officials is ramping up, and that Mueller could be close to subpoenaing witnesses for documents or testimony.

According to Reuters, grand jury subpoenas have been issued related to a June 2016 meeting at Trump Tower with Donald Trump Jr., former Trump campaign chairman Paul Manafort, and Jared Kushner, Trump's adviser and son-in-law. The three met with a Russian lawyer, Natalia Veselnitskaya, after Trump Jr. was promised damaging information on Hillary Clinton.

Mueller's spokesman declined to comment, and Ty Cobb, special counsel to the president, said he was unaware that Mueller took this step.

"Grand jury matters are typically secret," Cobb told the Wall Street Journal. "The White House favors anything that accelerates the conclusion of his work fairly. … The White House is committed to fully cooperating with Mr. Mueller."

In a statement, White House press secretary Sarah Sanders reiterated that former FBI Director James Comey said three times that Trump is not under investigation, and there is no reason to believe that's changed.

Deputy Attorney General Rod Rosenstein appointed Mueller to oversee the Russia probe in May, just after President Trump abruptly fired Comey.

Since then, Mueller has picked at least 16 lawyers to assist in the probe, which includes an investigation into Michael Flynn, Trump's former national security adviser.

A federal grand jury in Alexandria, Virginia, is currently looking into Flynn and his foreign lobbying.

Mueller's most recent hire came earlier this week when he asked Greg Andres, a former Justice Department official who most recently worked as a white-collar criminal defense attorney, to join the team.

It's unclear how long Mueller's investigation would take, but the president has taken to dismissing the probe as a "witch hunt" and "fake news."

Mueller's investigation appears to be wide-ranging. Last month, Bloomberg reported Mueller has begun looking into Trump's business transactions. The former FBI director is also examining whether Trump obstructed justice in trying to curb the investigation.

Trump has also criticized Mueller and his team of lawyers, calling into question their ability to remain impartial given previous campaign donations to Democrats, including Clinton, and privately inquiring about his ability to fire Mueller. The president's comments have led Republicans and Democrats to fear that Trump may take this step.

In an effort to provide Mueller and future special counsels with protection, Sens. Thom Tillis, R-N.C., and Chris Coons, D-Del., introduced legislation Thursday that would allow a special counsel to challenge his or her firing before a three-judge panel.