Special counsel Robert Mueller has impaneled a grand jury as part of his investigation into possible collusion between President Donald Trump’s campaign and Russia in last year’s election, sources tell the Wall Street Journal.

The report follows Mueller stepping up the probe by hiring several high-powered lawyers. Reuters reported earlier this week that Greg Andres, a former Justice Department attorney who specialized in corruption and bribery cases, joined the investigation.

Grand jury subpoenas related to a meeting attended by Trump Jr., the president’s eldest son, and a Russian lawyer have been issued, sources told Reuters. Trump Jr. agreed to the meeting after being promised information that would “incriminate” his father’s opponent, Hillary Clinton, and was part of the Russian government’s effort to benefit Trump.

A source familiar with the matter told Reuters that Mueller was investigating whether anyone at the meeting or affiliated with the Trump campaign encouraged Russia to release damaging material it had collected on Clinton. Another told the outlet that Mueller was trying to determine if Trump had been informed of the meeting ahead of time, or if he was briefed after it took place.

The president has strongly denied having had any knowledge of the meeting, but has not faulted his son for attending it.

Mueller’s probe is one of several government investigations into possible collusion. He is also investigating whether Trump obstructed justice.

Assembling a grand jury marks a major step, as it suggests an aggressive and prolonged investigation. Grand juries can be used as legal tools in cases when there is the possibility of criminal conduct, legal experts say.

The grand jury, which will be based in Washington, D.C., does not necessarily signify that criminal charges will be brought. But it means it can hear testimony from witnesses under oath, and that the investigation can issue subpoenas to obtain documents.

Trump has criticized the investigation as “a witch hunt,” and hinted at removing Mueller. Meanwhile, there has been bipartisan support for legislation to protect Mueller from being fired by Trump.

Deputy Attorney General Rod Rosenstein appointed Mueller in May, after Trump fired then-FBI Director James Comey amid the agency’s investigation into Trump’s campaign. According to Comey, Trump had told Comey to pledge his loyalty to him, undermining the independence of the FBI. He also asked Comey to lay off the FBI’s investigation into former national security adviser Mike Flynn, who lied about his contacts with Russian officials.

Last month, in an interview with The New York Times, Trump warned Mueller to not investigate his finances. Soon after, several sources reported that Mueller had begun including Trump’s finances as part of the probe.

White House press secretary Sarah Huckabee Sanders said Thursday in a statement that Trump himself was not under investigation. She referred to Ty Cobb, a member of Trump’s outside legal team representing him in the Russia investigations.

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.” Former FBI Director Jim Comey said three times the President is not under investigation and we have no reason to believe that has changed.

Jay Sekulow, another member of Trump’s legal team, similarly told Fox News that “we have no reason to believe that the president’s under investigation here.”

Sekulow downplayed the news of a grand jury as “not an unusual move,” calling it “standard prosecutorial approach.” He also suggested that the investigation will be resolved quickly, saying he thinks Mueller “will move expeditiously through the process” with the grand jury in place.

He also affirmed that Trump does not plan to remove Mueller.

“The president is not thinking about firing Bob Mueller,” he said. “This speculation that’s out there is just incorrect.”

This article has been updated throughout with more context about grand juries and Mueller’s investigation and response from the White House and Trump’s outside legal team.