And by mystery I mean just that: I'm preparing to run my PCs through a mystery adventure, wherein they will need to discover the reasons behind the protection being given to a small village against the wrath of two ogre-magi.

The secret is a very Lovecraftian/Gothic one: a few of the townspeople have been sacrificing children to an evil god to gain protection for the town. (They've been doing this for years, its just recently that there has actually been a threat to them).

At any rate, I've never really run a detective/mystery style adventure in my many years as a DM, as I tend to put a focus on political/character ineraction/sometimes combat. But never mystery. It's odd that I haven't before, but the point is:

I'm looking for some advice. Some suggestions. How to make it believable/workable. I think I'll be able to do it pretty well, as I'm good with realistic NPCs, but I was wondering if anyone had some helpful tips to throw in.

Thanks, and if the mods want to move this into the DM section that's alright, I'd just like to get a bunch of responses first, and it is a pretty general question.