Copied from somewhere else I posted it:
My party was investigating a serial killer who was petrifying people throughout town.
Each victim had a pair of puncture wounds on their neck. We immediately thought vampires for some reason.
The puncture wounds were eventually found out to be syringe-like.
We would bring victims to the town coroner to ask for his expertise in identifying what is happening to the victims.
He mentioned there was a chemical mixture that had this effect when mixed inside a body.