Victims of the Polar Night

In a distant polar city, an emergency occurred – a car fell from a bridge into the river. While saving the people who had gone under the icy water, divers discovered the body of a girl tied to a piece of concrete at the bottom. It turned out that this was the girl who had disappeared without a trace six months ago. Investigators Igor Omelin and Kirill Smirnov established that several other girls had also gone missing in the city at the same time. By a strange coincidence, all of them were donors at the local hospital. Is this just a simple coincidence or a thread leading to the unraveling of what happened?