DOJ is supposed to be the "law firm" for the regulatory agencies of the federal government. I.e., if the agency was not one that was an individual LEO type, DOJ handled ALL of our agency cases.
So the agency program staff would compile their investigative evidence. would write up their reports, and then would provide recommendations (with citations back to the applicable law/statutes that the agency covered), hand it over to our own agency's legal group (who would clean up the citations and format it), and then DOJ took it from there.
They even gave us templates to use to fill out to make it easier for them to get started on our criminal or civil actions. We would have occasional interactions back and forth with whoever was assigned the case, and particularly if the case ended up going to trial and the agency staff would have to testify (rare as most settle and/or comply before getting that far).
They have now gutted DOJ so who knows what is going on there. 
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