Different investigation teams could be brought in for the next 10 years, and unless some sketchy unheard audio like "this kid looks foreign, kill him" comes into play, the same conclusion is going to be reached time and time again.
I have several pellet pistols, some of which are modified to produce projectile speeds that could kill someone. While the rounds are small and top speed is maybe only 720-750 fps at best, if I hit you in the right spot, your dead. When I am transporting these, there is no Co2 cartridge in them, they are unloaded, and they are in a lock box in my trunk. I treat them like a real fire arm. Also, in the even I get pulled over and for some crazy reason have my car searched, hopefully the police will see the safety measures and don't think I am planning a fake bank robbery or something. Maybe this event will teach kids that even 'toy' guns can be a real threat and should not be treated as toys, and if a cop tells you multiple times to do something you do it.
I know the reports said that the police ordered the kid several times to put the weapon down, did they say anything about opening fire on him if he didn't? Not that I think that matters, but I can see a good lawyer being able to use that in court. I don't think it would hold up of it were a 21+ year old, but against a 15 year old, it might have some weight to it.