The District Attorney can request the California Attorney General's office to do the investigation into Robert Garth's death .* In the Daniel Sylvester taser execution in Del Norte, DA Riese requested the AG conduct the investigation. This has at least provided the appearance of more impartiality.
*Not sure what sort of pressure would be needed on Gallegos to request the outside investigation; not sure how much better an AG investigation is (the only satisfaction we got from it in Del Norte was that there are permanent records at the state level and that Del Norte had to squirm a bit. Still, that's something.). The only benefits I can see for pushing for external investigations is that the AG's office may eventually become so tired of doing them, stronger local internal oversight policies may be imposed and "shoot-first" behavior may be reduced, if they think they'll have to answer to DOJ agents.