Grand juries are members of the public who meet in secret to hear evidence. Grand jury subpoenas are the primary method by which prosecutors gather evidence in criminal investigations, and evidence gathered by one grand jury is easily movable to another.
Mr. Mueller is also investigating a June 9, 2016, meeting set up by Donald Trump Jr., the president’s eldest son, based on the expectation of getting damaging information about Hillary Clinton from a Russian lawyer. Mr. Manafort and Jared Kushner, the president’s son-in-law, also attended the Trump Tower meeting.
The younger Mr. Trump has said that the meeting did not produce any valuable information, and that he had no further contact with the Russian lawyer, Natalia Veselnitskaya.