I think grand juries are designed to indict suspects on preliminary evidence. If an indictment results, a warrant for arrest is issued. Congress would not be involved in this matter if a US Attorney is involved (i.e., Dept. of Justice =/= Legislature).
OK. But do you agree that "there must be a pending suit in a civil matter or charges filed in a criminal matter before a subpoena can be issued" is completely incorrect?
No, I do not agree.
I will graciously concede the argument and admit that I'm wrong if you can show me a rule of court procedure that shows me I'm wrong (no ill-will or ego, here). Until then, I'm going to go with what I know from my own experience as an attorney who practices a fair amount of civil litigation.
Your experience practising civil litigation won't give you much insight into how grand juries work. But presumably you went to law school at some point...