It happens. Dogs lose a scent, or they get confused, or whatever. Bloodhounds are good, but it's not exactly rare for people and animals to elude them.
The boy was found at the overpass, and he died from falling. The overpass is so close to his house that he could easily have walked there. There are plenty of reasons a 13 year-old boy might have been playing around in such an area without anyone knowing it.
That it was an accident is a reasonable thing to believe, is what I'm saying. Without a videotape or something it's always possible this was some sort of crime that was concealed, but there's no evidence of this. In fact, the signs are otherwise. It seems strange that a killer would try to murder the kid by climbing up to the overpass-- with the boy-- and then throwing him off. What if he was seen by passing traffic? What if he couldn't climb up with a struggling boy in his grip? Etc.
I'm really sorry about the boy. If you're from the area, you have my condolences.