Hey wrk, sorry to hear that about your dog, and your experience with the cops. The cops did all they can do really. Pictures prove only that an event happened, they don't prove the kids in the Escalade did the shooting. Also, full on CSI: ballistics, trajectory, evidence recovery, would only solidify the event happened, and it would be a Wonderful Wizard of Oz miracle to connect all that to a group of wanna bes in a "similar looking" vehicle without a gun. Short of finding the gun (in the vehicle or on one of the kids), the ammo, and the recently fired pristine bullet that penetrated your pup to match it to the gun would be the only thing that would prove those particular kids did the shooting. And, like the cops said, there is no point. Hate to say this, but a dog's life is not the same as a human's life, the kids were probabaly underage and so the cops won't waste the time....unless they had the gun, ya know.
If you look at it from a different perspective though, at least the cops did a little something. They seemed quick to respond, and they followed up (even though it pissed you off). In a bigger city/metropolis you would be extremely lucky to get a timely response....for a shot dog. They also found who YOU or YOUR MOM thought might have done it, but there was no probable cause, and other than: circumstance, proximity and similarity, unfortunately that's, I'm sorry to say, not enough Brother.
And, whether you think so or not, the cops are really on your side, and the cops probably know in their gut that those kids shot your pup, and they want to hook those kids up too, believe me they do, but they can't do anything without the gun, so......anyway
Again, sorry this happened, have some faith man, karma is a bitch and it will catch up them....eventually.
Hope the pup recovers well.