re: What is the most chilling unsolved murder mystery you know?

In 1973, when I was in high school, a young, 5-year-old girl by the name of Anna Waters disappeared without a trace in a very rural and sparsely populated area of the county I lived in, just south of San Francisco. Fast forward to 2006. I was assigned her cold case. My partner and I worked the case on and off until we retired.

Had she been swept away in a fast-running creek near her rural home? Was she dragged off by wild animals? Was she abducted by Edmund Kemper or Herbert Mullins, two notorious serial killers working the area at the time? (We interviewed them both, we did not believe they had anything to do with her disappearance.). Had her estranged, biological, schizophrenic father kidnapped her? Would any of the other approximately 150 leads pan out? Would any of the numerous DNA tests conducted in both the USA and Canada lead us to Anna?

Once it was put out in the press that we had reopened the case, we received roughly 4-6 calls every day for about a month and a half, from people claiming to be Anna (DNA test were conducted on all), or people claiming to know who had taken her. We followed up every lead with an interview (If people were still alive or could be found.).

The answer to all was either no, or we don't know. This young girl, within the span of roughly 3-5 minutes, vanished from the face of the earth without a trace. The case remains open.