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. Earlier this week, Reuters reported that Greg Andres, a former Justice Department attorney who specialized in corruption and bribery cases, joined the investigation.
Sources tell Reuters that "grand jury subpoenas have been issued related to [a] June 2016 meeting between Donald Trump Jr., Russian lawyers and others." Trump Jr., the president's eldest son, met with a Russian lawyer last summer after being promised information that was part of a Russian government effort to benefit Trump and would "incriminate" Trump's opponent Hillary Clinton.
Mueller's probe is one of several government investigations into possible collusion. In addition, it is investigating whether Trump obstructed justice.
The assembling of 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.
While the grand jury, which will be based in Washington, does not necessarily signify that criminal charges will be brought, 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 FBI'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 began including Trump's finances as part of the probe.
This article has been updated throughout with more context about grand juries and Mueller's investigation.