Rob.. Each Federal Judicial District (I am in the Southern District of Texas) has it's own United States Attorney who is appointed by the President and serves "at his will", so they really are not responsible to The Attorney General/Eric Holder. Some have been of office for quite some time and others are changed with whoever is POTUS at the time. So, you will find a big difference in how Federal law is administered from Federal Judicial District to Federal Judicial District.

The United States Attorney General is over the Department of Justice with all it's divisons and task forces. He really has very little to do with the various United States Attorneys. His own folks can and do bring prosecutions from their various divisons such as Civil Rights, etc, etc. But the run of the mill criminal prosecutions are handled by the United States Attorneys in the various districts who don't answer to Holder. Of course Holder can ask the President to remove one or more of them. Bush removed some and remember the stink that caused.