First, my career path is: Beat cop/firefighter, then moved to arson investigation where I am now.
Working on the timeline of this incident and assuming it’s correct SBNT an officer-involved shooting (as opposed to a weapon discharge where nothing was damaged) would follow like this. Incident occurs, command staff and IA arrive (sometimes they’re the same people) begin the investigation immediately, as in while the scene is being taped and before all of the evidence is gathered. The officer, for his own safety, gives up his weapon (again, to ensure he was using the right rounds, that the weapon was well maintained and functioning properly and to be recertified for use. Remember, this weapon is public property). He is asked a litany of questions (you’ll forgive the use of the masculine here) by command and/or IA is sent to critical-incident stress debreifing, then monitored by department personnel (should those resources permit). This is all done in the 24-72 hours after the shooting, because the truth is, the elected officials want this to go away, there’s no dallying. Presuming that the officer is union (which most are) he’s entitled to have a union attorney present during the aforementioned questioning. The initial findings of the investigation are presented to command staff and/or a board of inquiry (or whatever liason body) for the next step. Concurrently, county or state officials, as impartial 2nd parties begin their own investigation. This does take a little longer, but just quitting the force doesn’t mean you get to skip away from the incident fingers-in-your-ears shouting LA LA LA.
Then, if there is any action to be taken, it is decided upon by the sharks, er, lawyers, and the command staff on behalf of the department. If he had stayed on, he would have likely (or at least should have been) in the station for the following 12 hours. In shooting incidents, people can have a sometimes seemingly schizophrenic reaction to killing another human being, debriefing is often key. All of this would be done within that 72 hour window. Charges might not be quick to come and frankly, you have more than just the officer, but the union who will represent the officer and has pockets as deep or deeper in some cases, than the people. Dragging him in on a bogus, hurried arrest allows union attorneys to tear the arrest apart and could set in motion a series of events that lead to a bad verdict or none at all. It’s not worth dragging him in right now. When they do, they’ll have everything they need to convict. Yeah, it’s likely a double standard, but it’s a necessary one, otherwise, you would, believe it or not, be a LOT less safe. If you start dragging cops in to jail everytime they shoot someone based on public pressure, you have a condition that puts hesitation in the mind of a cop, and that hesitation will get you as an officer or worse, an innocent person killed.
Having looked at another version of the vid on youtube from the KTVU report, I can guess what happened; shit was getting out of control, Grant was starting to get shitty with the cops, the rookie (2 years on the job) goes to some effort to ready his weapon, I suspect he’s trying to force some situational awareness (read: press gun to subject, hollywood style) on Mr. Grant when the discharge happens. Always a bad move, never in a situation like that do you unholster your weapon, someone in his case, who was not a cop had ample opportunity to remove it from his grasp. The key thing to watch for is that unless you’re firing for your life, you don’t do so one-handed like he did. Straight, safe, clean shots are fired, whenever possible using two hands. He wasn’t firing for his life, he wasn’t blind firing to lay down cover, he wasn’t shot in his strong hand, he was acting like a schmuck and this poor kid paid for it. It’s not murder, but it’s negligence, in my opinion, of course.