The grand jury established by Special Counsel Robert Mueller to help him with the Russia investigation has issued subpoenas related to the infamous June 2016 meeting held at Trump Tower between Donald Trump Jr. and Russian operatives, Reuters reported Thursday.

News of the subpoenas broke shortly after the Wall Street Journal reported that Mueller had established a grand jury in Washington, D.C. — an indication that his probe is gaining momentum rather than slowing down.

A grand jury has special authority to pursue indictments, subpoena documents, and put witnesses under oath.

Details surrounding the direction and nature of the investigation have also started to trickle out. Federal investigators are reportedly digging into President Donald Trump and his associates’ financial ties to Russia, which are proving to be “the most fertile avenues” for moving their probe forward, according to CNN. Even so, CNN quotes two law enforcement sources who say the probe has also extended to possible financial crimes unconnected to the 2016 election.

Legal experts consulted by the Journal said that while impaneling a grand jury doesn’t necessarily signal criminal charges are coming, it does suggest how seriously Mueller is treating the investigation and allegations.

Before Mueller was designated as Special Counsel (following Trump’s abrupt dismissal of former FBI Director James Comey), federal prosecutors had impaneled a grand jury in Alexandria, Virginia, to help their criminal investigation of Trump’s former national security adviser, Michael Flynn.

President Trump has repeatedly sought to undermine the credibility of Mueller, saying he’s leading a “witch hunt” and is secretly allied with Trump’s former Democratic rival Hillary Clinton.

Many of the investigators and prosecutors hired by Mueller fueled speculation that he was giving careful scrutiny to Trump’s finances. For example, in June Andrew Weissman left his position as chief of criminal fraud at the DOJ when Mueller offered him a senior management role on the special counsel team. Weissman is an expert in international fraud and organized crime, famed for his role in combating mob activity on Wall Street. Other additions to the team include Jeannie Rhee, white-collar crime specialist and former senior adviser to former Attorney General Eric Holder, and most recently Greg Andres, a longtime white-collar defense lawyer who most recently served as deputy assistant attorney general in the DOJ’s criminal division, overseeing the fraud unit and the program targeting illegal foreign bribery.