By Karen Freifeld and John Walcott
WASHINGTON (Reuters) - Grand jury subpoenas have been issued in connection with a June 2016 meeting that included President Donald Trump's son, his son-in-law and a Russian lawyer, two sources told Reuters on Thursday, in a sign that special counsel Robert Mueller's investigation is gathering pace.
The sources added that Mueller had convened a grand jury in Washington to help investigate allegations of Russian meddling in the 2016 U.S. election.
Russia has loomed large over the first six months of the Trump presidency. U.S. intelligence agencies have concluded that Russia worked to tilt the presidential election in Trump's favor. Mueller, who was appointed special counsel in May, is leading the probe, which also examines potential collusion by the Trump campaign with Russia.
Moscow denies any meddling and Trump denies any collusion by his campaign, while regularly denouncing the investigations as political witch hunts.
Mueller's use of a grand jury could give him expansive tools to pursue evidence, including issuing subpoenas and compelling witnesses to testify. The impaneling of the grand jury was first reported by the Wall Street Journal.
A spokesman for Mueller declined comment.
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."
U.S. stocks and the dollar weakened following the news, while U.S. Treasury securities gained.
News last month of the meeting between Donald Trump Jr. and a Russian lawyer who he was told had damaging information about his father's presidential rival, Democrat Hillary Clinton, fueled questions about the campaign's dealings with Moscow.
The Republican president has defended his son's behavior, saying many people would have taken that meeting.
Trump's son-in-law and White House senior adviser Jared Kushner and former Trump campaign manager Paul Manafort also attended the meeting.
Ty Cobb, special counsel to the president, said he was not aware that Mueller had started using a new grand jury.
"Grand jury matters are typically secret," 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."
John Dowd, one of Trump's personal lawyers, said: "With respect to the news of the grand jury, I can tell you President Trump is not under investigation."
A spokesman for Manafort declined to comment.
Lawyers for Trump Jr. and Kushner did not immediately respond to requests for comment.
One 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.
(Additional reporting by Noeleen Walder, Jan Wolfe, Anthony Lin, Jonathan Stempel, Tom Hals, Julia Ainsley and Joel Schectman; Writing by Frances Kerry and Phil Stewart; Editing by Bill Trott and Peter Cooney)