Another commenter posted a link that reported they did find gunshot residue: https://www.fox5atlanta.com/news/autopsy-cop-city-protester-...
In other words: the accused are investigating themselves, and have produced a report that suggests that they were justified in killing a climate activist. I think that's a pattern we'd normally treat with extreme skepticism; it's unclear that GBI has earned an exception to that.
No; the Georgia State Patrol is the primary agency that was involved. It's a different agency.
Good news then, the State DA recused and GBI is a different agency than the state police.
(It hopefully isn't controversial to say that, as a general principle, entities in positions of exceptional public trust or interest should not be tasked with enforcing or regulating themselves.)
What is the role of a state level investigation bureau if not to investigate state-level issues involving state agencies?
However, to answer the following (which isn't what I meant, but I think is interesting):
> but assuming it’s a private firm would you not have the same criticism that it’s not a neutral third party and is biased towards whoever hired them?
I think the answer to this is similar to the answer found in arbitration: arbitration is typically performed by a well-known firm, and that firm is obligated through contract to perform fair proceedings. In other words: there are civil penalties on the table if the independent party does not execute their job fairly.
That being said, I think an unrelated but still governmental investigation arm would be sufficient here.
> What is the role of a state level investigation bureau if not to investigate state-level issues involving state agencies?
You can find the GBI's services listed here[2]. They have a lot of responsibilities, most of which aren't tied to investigation of local police. I think it'd even be fair to say that they aren't particularly equipped to investigate local forces.