The special counsel investigating ties between the Trump administration and the Kremlin, along with Russian interference in the US election, has started using a grand jury in Washington, reports have revealed.
Robert Mueller, who is conducting the investigation, has issued grand jury subpoenas related to Donald Trump Jr's meeting with a Russian lawyer last summer.
The move signals that Mueller's probe is ramping up as he takes a more aggressive approach to gathering information on possible collusion between the Kremlin and Trump's campaign team.
Prosecutors use grand juries to compel witnesses to hand over relevant documentation and to testify before the counsel. The juries are composed of between 16-23 members of the public who may ask witnesses questions.
After hearing witness testimonies in secret, grand juries decide whether or not to officially accuse a criminal defendant and force that person to stand trial.
A grand jury is not a court of law and jurors do not decide the innocence or guilt of a potential defendant. Charges will not necessarily be brought: the role of the jury is to decide whether an arrest is warranted and a further investigation is necessary.
Grand jury investigations can take months or even years as jurors work to gather enough evidence to compile a case.
Analysts say that Mueller's use of a grand jury shows that he is taking control of the investigation.
Mueller is reportedly investigating whether Trump knew of his son's meeting with Russian officials in the run-up to the election, according to Reuters.
The president's son was told Russian lawyer Natalia Veselnitskaya was willing to share "information about illegal campaign contributions to the Democratic National Convention" with the Trump campaign, ahead of their meeting at Trump Tower in July 2016.
Trump Jr has said he did not tell his father about the meeting. Mueller will be seeking to determine whether Trump Sr had prior knowledge of the meeting or whether he was briefed on it afterwards.
The president has called the investigation into his administration's ties with Russia a "witch hunt" and a "hoax" that is being used to "demean the United States."
"The Russia story is a total fabrication," he told a rally in West Virginia on Thursday evening (3 August). "Most people know there were no Russians in our campaign. There never were. We didn't win because of Russia, we won because of you," he said.