In the headline reports there is someone termed a 'special prosecutor'. The serious 'newspaper of record'
The Times tells what he is doing and why but has evidently assumed that its readers are familiar with the job.

(Sometimes they expect too much. The other daily paper we take would only explain it if the explanation could be illustrated with a picture of a topless girl, which event has not yet happened; believe me I've looked

)
So: How is the prosecutor appointed and by whom? Is he a regular employee in some prosecution department who is simply assigned to the case? Who decides what cases are worthy of his attention? How are his actions instigated: there seems to have been no police involvement in this case of Libby and the spy, so does he act on reference by a member of the public or somebody in , or some committee in, the legislature?
He is reported to have questioned potential witnesses and suspects. This makes him sound like a French magistrate, who takes over the investigation of crimes and possible crimes and summons anyone he or she thinks may help (These people are obliged by law to answer the questions; silence is not a response that leaves them at liberty )
A subsidiary question is that of the Grand Jury.At one time, in the distant past in England,it was the local people who formed an investigating body, a grand jury. As soon as there was any complaint of crime they had the suspect brought before them and they decided whether the case was fit for trial by the visiting circuit judge ( Judge of assize) with another jury, whenever he would reach the county town . Originally they were more of investigating body than simply judges of whether there was a prima facie case prepared for them and presented by some prosecuting authority.
Here , in the instant case, there is mention of a grand jury. Who calls it, who decides on the need for it and when? Does it have any role in summoning evidence or is it entirely passive? Other cases of an obvious criminal nature, with considerable police involvement, , such as Michael Jackson's, showed this kind of jury to be just like the grand juries we last had (they were abolished in the 1960s). The police investigated, a prosecution department assembled the evidence and presented it and the jury decided simply whether there was a prima facie case. (This role is now taken by magistrates)