When a murder leaves traces but little certainty

The Könizbergwald murder shocked Switzerland, which is hardly known for a shortage of orderly procedures and tidy assumptions. A mother was convicted of killing her eight-year-old daughter with a stone in the forest, yet the case did not rest on a simple smoking gun. It became an indizienprozess, because the traces at the scene were abundant but not self-explanatory. That is the part the public tends to miss when it imagines DNA as a kind of moral lie detector.

At a suspected crime scene, investigators first try to collect as much information as possible without deciding too early what it means. Jörg Arnold, deputy director of the Forensic Institute Zurich, says the aim is to enter such a case with an open mind. The site is documented, the deceased is examined, and then the search begins for blood traces, DNA, hair, fibres and objects that might matter later. The problem is not finding material. The problem is deciding which fragment belongs to the crime and which one merely belongs to life before the crime.

In Könizbergwald, two things made that distinction harder. The mother and daughter knew each other before the killing, which in forensic language means there was a form of scene entitlement: traces between them could also come from ordinary contact, not necessarily from the attack itself. On top of that, there was a large amount of the victim’s blood at the scene. That matters because blood carries a great deal of genetic material and can drown out weaker traces from the perpetrator.

When there is much more victim DNA than perpetrator DNA, the latter can be pushed aside in the laboratory process. Arnold compares it to a landslide that covers whatever the offender left behind. The difficulty grows again when victim and perpetrator are of the same sex, because one common method in serious violent and sexual cases is then unavailable: the targeted search for male DNA on the Y chromosome. In mixed traces, that can help expose a male offender even when female victim DNA is overwhelming. Here, that shortcut was not available.

The scene itself can also work against investigators. Moisture, heat and UV radiation break down biological material over time, especially outdoors. How much this matters depends on the case and on how long it takes before the body and the scene are discovered. In other words, nature does what it usually does: it interferes without asking for permission.

The lesson from Könizbergwald is not that forensic science failed, but that it is often asked for certainty where only interpretation is available. Spuren do not speak for themselves, however much modern technology is supposed to flatter that idea. Investigators must resist the temptation to lock onto a theory too early. Only the combination of different traces and examinations can reconstruct a credible picture of what happened. The public may prefer clean answers; the evidence, as usual, is less cooperative.

Written by Christiane Hofreiter christiane.hofreiter@alpineweekly.com