Six buried arms and staged bodies
Investigators find six severed left arms buried in a forest clearing. Five belong to girls recently abducted, and the sixth belongs to an unidentified girl who may still be alive.

4 ideas
Investigators find six severed left arms buried in a forest clearing. Five belong to girls recently abducted, and the sixth belongs to an unidentified girl who may still be alive.
The 'whisperer' never needs to kill anyone himself. He finds people who already carry suppressed violent impulses and uses suggestion and manipulation to give them permission and a push to act. His guilt sits in the steering rather than the act, which makes him nearly invisible to investigations built around finding the person who did the deed.
A staged body can be read as a communication rather than as evidence left behind. Where and how it is placed tells investigators what the sender wants them to see, including other people's hidden guilt. Asking 'what is this scene trying to tell me, and why me?' produces different leads than asking 'what traces did the killer leave?'
The criminologist has the team give the unidentified killer a human name, 'Albert.' This turns a shapeless monster into a person with motives, habits and limits that can be reasoned about. The name also counters the fear and myth that make an unknown adversary seem all-powerful and paralyze an investigation.