First, we need to figure out what the normals know in general about the supernatural.
Until six months ago, little beyond what you'd expect of pop culture. Lately, there's all sorts of crazy semi-accurate rumors floating around, but little concrete knowledge if any.
Second, we need to figure out the local supernatural scene, its power players, its small fry, and the relationships between all of them. Also, we need to subvert at least a couple to become helpful to us.
Well, the relationships are quite straightforward: Everyone hates everyone else.
Third, we need to figure out what government forces, normal and supernatural (if the supernatural has governments), are actively interacting with the other side. This is really important, as it can indicate the extent of the interference.
???
Then we can start looking for the cause.