Irene Huss 8: The insidious net

A bicycle messenger sees a girl being brutally assaulted and dragged into a van. She immediately calls the police, but unfortunately the girl is found too late. It turns out that the murdered girl has been the victim of a serious sexual crime. Detective Inspector Irene Huss and the rest of the team begin to look for the murderer, but have very few clues to go on. When another young girl is murdered in a similar way, the team realizes that they are dealing with a sexual predator. How does he get in touch with the girls? Why do they agree to meet him? Why are they wearing special underwear?