Grand jury issues subpoenas in connection with Trump Jr., Russian lawyer meeting

Robert Mueller pauses after making an opening statement at the US Senate Judiciary Committee hearing on Capitol Hill in Washington, DC, US on June 19, 2013. [Photo/Agencies]

The sources added that US Special Counsel Robert Mueller had convened the grand jury investigation in Washington to help examine allegations of Russian interference in the vote. One of the sources said it was assembled in recent weeks.

Russia has loomed large over the first six months of the Trump presidency. US 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.

At a rally in Huntington, West Virginia, on Thursday night,Trump said: "Most people know there were no Russians in our campaign. ... We didn't win because of Russia. We won because of you."

Mueller's use of a grand jury could give him expansive tools to pursue evidence, including issuing subpoenas and compelling witnesses to testify. The Wall Street Journal earlier reported a grand jury was impaneled.

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.