A grand jury is a group of ordinary citizens who, working behind closed doors, considers evidence of potential criminal wrongdoing that a prosecutor is investigating and decides whether charges should be brought. "This is a serious development in the Mueller investigation," said Paul Callan, a former prosecutor. "Given that Mueller inherited an investigation that began months ago, it would suggest that he has uncovered information pointing in the direction of criminal charges. But against whom is the real question." A lawyer for Trump, Jay Sekulow, appeared to downplay the significance of a grand jury, telling Fox News: "This is not an unusual move." US stocks and the dollar weakened following the news, while US Treasury securities gained. It was not immediately clear to whom subpoenas were issued and the sources did not elaborate. Some lawyers said it would put pressure on potential witnesses to cooperate with Mueller's investigation. "When someone gets a subpoena to testify, that can drive home the seriousness of the investigation," said David Sklansky, a professor at Stanford Law School and a former federal prosecutor. In 2005, a grand jury convened by Special Counsel Patrick Fitzgerald returned an indictment of Lewis "Scooter" Libby, a top aide to then-Vice President Dick Cheney.The Mueller Grand Jury situation is a good thing. Everything now is on the record. No more speculation in official quarters. @billoreilly
— Robby Ball (@perfectsliders) August 4, 2017
'Not thinking of firing Mueller'
Trump has questioned Mueller's impartiality and members of Congress from both parties have expressed concern that Trump might dismiss him. Republican and Democratic senators introduced two pieces of legislation on Thursday seeking to block Trump from firing Mueller. Sekulow denied that was Trump's plan. "The president is not thinking of firing Bob Mueller," Sekulow said.#MUELLER IMPANELED #RUSSIAPROBE #GRANDJURY: INITIATES PUBLIC REVIEW & #PROSECUTORS MAY DEMAND #EVIDENCE NOT VOLUNTARY PROVIDED -- #Resist https://t.co/qjpCAXhw1f— Jayce Kingstone (@JayceKingstone) August 3, 2017One source briefed on the matter said Mueller was investigating whether, either at the meeting or afterward, anyone affiliated with the Trump campaign encouraged the Russians to start releasing material they had been collecting on the Clinton campaign since March 2016. Another source familiar with the inquiry said that while the president himself was not now under investigation, Mueller's investigation was seeking to determine whether he knew of the June 9 meeting in advance or was briefed on it afterward. Reuters earlier reported that Mueller's team was examining money-laundering accusations against Manafort and hoped to push him to cooperate with their probe into possible collusion between Trump’s campaign and Russia. It is not known if the grand jury is investigating those potential charges.