It's actually a loophole in the Constitution.
The Fifth Amendment says that nobody can face a criminal trial without an indictment from a grand jury.
The Sixth amendment says that all trials are by jury, and must be held in the district where the crime occurred.
The Constitution does not say that the grand jury must be in the district where the crime occurred.
-PJ
Well, it would seem to be in conflict with the spirit of the Constitution if not with the letter.