As far as Washington, DC and our Grand Jury system, I can assure you, from having served once as a Grand Juror for my "month", and knowing lots of folk who have similarly been called to serve, that this is how we do it here.
There are ALWAYS at least 5 sitting Grand Juries in Washington, DC, each with its primary responsibilities, generally being along the lines of the type of crime being investigated in any particular case. Multiple cases are floated, different Prosecutor teams appear before the sitting Grand Juries, interview witnesses, some of whom have just visited a suspect in the DC Jail, and record the testimony of such witnesses, often I suspect limiting future testimony of that witness...
"What did you talk about?" ... "nuttin'"... "Did Mr. X sleep at your home last Friday night?"... "nah" or " I dunno "...How well do you know the suspect? "Not much". Did he ask for for anything?... "Ya, underwear..."
Compelling stuff, right? Just saying, we have Grand Juries coming out our secret places. So many that often when some big political issue may arise, the SC will convene "his" Grand Jury outside of Washington DC proper, often in Virginia... All depends on the SC of course, but we are not strangers to Grand Juries, and the Grand Jurors who eventually may vote out the requested charges, are not always the same Grand Jurors who heard the original testimony, or the many other testimony clips that have been read into the record.
One major use of Grand Juries is to get people ON the Record, early and often, and take away (in my opinion) potential alibi use of said "witnesses" later. The stuff we saw in my Grand Jury stint was Capitol Offense, Major Crimes, mostly and yet we often observed witness interviews regarding some 'dance club' or a purse snatching from Realtors showing a home, kind of thing, when we were the only Grand Jury available for the Prosecutor to get a witness testifying that particular day. Getting them on the record, early, was the goal.
I expect if one wishes to Google: Wash DC GrandJury Service, you can get all this without the first hand reporting I just gave.
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Athos is a TROLL, and so he regards me as a troll, and you... by the way.