I don't understand what your point is and I think we may be talking about two different incidents.
We aren't talking about two different incidents, because I'm not talking about any specific incident.
I'm talking about your casual commentary that "physical evidence proves" whatever you think it has proven.
If I shoot someone, because I have been in the "education by incarceration" system I have a pretty good idea what evidence can be used to convict me, and I've done a fair amount of research on what to do about that evidence. I will have a fairly short time to exercise that knowledge, because when the first cop sets foot in the area I need to not be there.
If a cop shoots someone they have a
ton more education than I have about evidence, and far more hands on experience with what can be done to dispose of it. They don't need to be in any hurry, because even though the scene of the shooting is going to be declared a crime scene,
no one on that scene is going to be investigating the shooting. They are going to be checking every possible shred of anything that might be evidence of the
incident offense that got the criminal shot.
Then there is the lab. Staffed by people who have to work with their 'brother officers' every day. They also are going to focus primarily on the incident offense. Unless there is some extraordinary public pressure applied their interest in investigating the shooting is minimal, at best. You talk about federal investigations in Ferguson, and by the time the first feds arrived the crime scene had been trodden by at least a score of FPD and StLouis County boots, and exposed to weather for most of a night and day. The only physical evidence of consequence was collected or trampled by the locals. So the feds work with what they get.
Then there's lawyers. I shoot someone, I call JR and hope he'll work pro bono. A cop shoots someone he has a defense team supplied by the cop union. I face a prosecutor who wants my scalp just on general principles even if they don't really think I shot anyone. The cop faces a prosecutor who may already have an established record of torpedoing his own cases when it comes to prosecuting cops. My prosecutor is going to drive the local cops to turn over every rock, the cop's prosecutor is going to tell prospective witnesses "have you really considered the consequences of testifying against a police officer?"
Bottom line, your casual acceptance of "well the truth came out and blah blah blah" about the Ferguson case, or
any case involving a cop, is just mind bending myopia.