WASHINGTON (AFP) - A grand jury plays a key role in the US judicial system when it comes to investigating potential crimes and in bringing charges.

Robert Mueller, the special counsel heading the wide-ranging probe into whether there was collusion between Russia and members of Donald Trump's presidential campaign, has impaneled a grand jury in the case.

CNN has reported, and other media have confirmed, that the grand jury has brought its first charges stemming from Mueller's probe.

It is not known who was charged or what they were charged with, but CNN said details could be made public as early as Monday (Oct 30).

Grand juries typically meet in the strictest secrecy and deliberations are confidential.

Trump has repeatedly denied any collusion between Russia and members of his White House campaign and denounced the probe as a "political witch hunt." The most important role of a grand jury is to determine whether the evidence presented merits bringing charges.

If so, an indictment is produced and a case may go to trial.

Even before that, though, grand juries have the power to issue subpoenas compelling witnesses to testify or for documents to be produced.

Grand juries do not determine guilt or innocence.

Rather, they decide whether probable cause exists that a potential crime has been committed and if charges should be brought.

The grand jury gets its name from the fact that it is often made up of between 16 and 23 US citizens, rather than the usual six or 12 for a jury trial.

Former US president Richard Nixon appeared before a grand jury for two days in 1975 - nearly a year after resigning the presidency - in connection with the Watergate scandal.

In August 1998, Bill Clinton became the first sitting US president to testify before a grand jury as he was being investigated for alleged perjury and obstruction of justice.