People have been mysteriously disappearing on a stretch of a road, for years. A policemen is asked:
-- So what's the theory?
-- Honestly, we don't know. Serial murder? Kidnapping ring?
Supernatural, TV series
An organized group of people involved in, in this case, kidnapping.
You can hear this in a few examples like also "smuggling ring." It implies there's a high level of organization with leaders, people who handle different aspects of this crime, low level workers involved, etc.