But what is the grand jury and how could its deliberations lead to the end of the political life of a president who is as popular in the polls as Bill Clinton?

Monica Lewinsky: Centre of the storm

Grand juries are a keystone of the US judicial process with two main roles:

They decide if there is 'probable cause' to believe a crime has been committed.

They are charged with protecting citizens by ensuring that unfounded criminal prosecutions are not pursued.

Unlike a trial jury, a grand jury does not determine on guilt, just whether there appears to be a case to answer.

Powerful investigative tool

A grand jury usually sits for 18 months and contains a minimum of 16 members but more typically 23.

Although they can be made up of anyone eligible to sit on a trial jury the panels tend to include more retired people because of the time the length of service required.

The president's bodyguard Larry Cockell was ordered to give evidence

Kenneth Starr is using two grand juries to investigate different aspects of the Monica Lewinsky affair. Each is equipped with powerful investigative tools; the most important of which is the power to issue subpoenas.

Normally, the prosecutor controls proceedings in private, no judge is present and witnesses, who are under oath, must testify without their lawyers in the room - although witnesses are allowed to ask for permission to leave the room in order to consult with their legal advisors. (President Clinton's legal team have negotiated to be present when questions are asked, to provide answers on videotape and to have proceedings conducted in the White House.)

And whereas trial juries purely listen to evidence, grand jurors can ask questions - and regularly do.

Fifth Amendment

Witnesses can plead the Fifth Amendment and refuse to answer self-incriminating questions.

But in such cases they may be granted immunity from prosecution - which obliges them to answer or be jailed from contempt. (Monica Lewinsky has been granted immunity in return for giving the grand jury truthful testimony about her alleged affair with the president.)

Rife with rumour

Linda Tripp speaks to the press after her grand jury testimony

Because grand jury proceedings are often long, complicated investigations and conducted in private, they are often surrounded by rumour, false leads and speculation.

That is one reason why prosecutors and jurors are legally prohibited from making public comments although witnesses are free to discuss their testimony.

At the end of the day the grand jury has the power to close the book on Mr Starr's investigation or decide if there should be another chapter.