My understanding is that there were more than two people firing the shots - after all, the two that have been arrested were fleeing together and were caught at the same location, so it's unlikely that they were shooting at each other.

As such, it's quite possible that the bullet that killed the girl (or those that injured the other bystanders, for that matter) weren't fired by either of the arrested youths, but by someone else entirely.

If it can't be proven that either of the arrested youths actually hit anybody, I think the most they can be charged with are weapons possession and reckless endangerment or something similar.