In Season 3 around episode 6 was when this really started to kick in ...
I think this is the best season so far in terms of going from curious, to interesting, to smoldering to on fire. Since I am not done with it yet, I don't know if the ending is going to be as good as the rest of it, or if it will be reliant on some gimmick, but these two guys really are good together.
Now, the other part of this is what Police Department in the whole country pays two guys to spend their entire careers on one single unsolved case? None, exactly, but overlooking that, and assuming that they must have also been doing other stuff, the threat of this story is pretty good, and they way they balance it over time and pepper it with the newer scenes of the effects of Alzheimer's ... this is a respectable piece of work ... very compelling to watch.