you have to talk to all three of the guardians Snake (the cat at the death valley , Thunder (the cat in the forest) and Mystic (the cat at the beach) (if you haven't fought against them, you'll have to do that first). they'll tell you clues about who the murderer is, and then you just talk to sunstar.
SPOILER ALERT!!! YOU HAVE BEEN WARNED!
sandstripe is the murderer, it's obvious