This doesn't make sense, the DA can't stop an FBI investigation.
DA stands for "district attorney", which is a state level position. The F in FBI stands for Federal, state DAs can't instruct the FBI on what not to investigate.
There is also no such position as "Chief".
The chain of command goes like this;
Director
Deputy Director
[CID Branch]
Executive Assistant Director
Assistant director
CID would be the branch investigating the situation in this VN and there is no DA anywhere that can instruct the FBI on what not to investigate.
Also any attempt to interfere with the investigation would be reported higher up the CoC which would trigger an internal investigation at a higher level.
So unless the claim is that some worthless
Hollywood Hillside fucks can control the ENTIRE FBI from the Director down, then none of this plot line makes any sense.
Now lets say that the Director is in their pocket, just for argument sake.
Then the situation could be reported to the AG, DOJ and DoNI.
There is no way to keep this from being investigated and the higher up it gets, the more out of control of the corrupt elements it gets.
Also every field office has a representative for the DoNI.
You can't buy off everyone and not have the situation go public, it isn't possible because if it was it'd already have been done.
I think the author is confusing FBI with local police.
If you want to make shit up so that your plot can work, then make it up, but don't use real agencies without knowing how it all works, it just looks dumb.
Loads of Authors seem to want to make the USA [or other western countries] a stetting for their story but don't actually know anything about how it actually is in the USA [or other western countries].