Transfer of Fibres onto Knife Blades in Stabbing Events: Distribution and Determination of the Stabbing Sequence

Knives are among the weapons most frequently involved in criminal cases. They represent the most encountered category of weapons in Swiss homicide cases (completed and attempted homicides considered) and are also frequently employed in assault cases, notably bodily injuries. Whenever a knife is involved in a stabbing event, DNA and fingerprints may be sought. When garments are damaged, fibres can also be investigated. Fibres from the victim's garments might transfer onto the blade of the knife used in the assault and can thus provide useful information to determine whether a particular weapon could have be used to stab the victim. This study simulates vertical stabbings into garments with the use of a special holding device. Different types of knives and blades straight or serrated were used as weapons. Two garments presenting different shedding capacities and garment structures were also considered for the simulations. The distribution of fibres transferred onto the blade (number and position) was recorded for each simulation performed. Sequences of stabbings into the two garments were also carried out to assess whether the order of the stabs could be determined. Several parameters were considered, notably the distribution of fibres transferred onto the blade. The transfer of fibres inside the stab damage ballistic soap was used in this study and on the area near the second damage was also investigated. This study provides new insight into the interpretation of fibres transferred onto knives after a single stab or a sequence of stabbings and into determining the stabbing sequence. Finally, the study brings some guidelines for the search and recovery of fibres on the crime scene and on the blades of knives

Les paramètres clés de l'interprétation des fibres textiles en sciences criminelles. Partie I: Occurrence et bruit de fond

Français: La question au centre d'une expertise forensique est la détermination de la valeur probante d'une correspondance entre une trace et un matériel suspect. Dans le domaine des fibres textiles, il s'agit de se prononcer sur la signification d'une non-différentiation entre le textile en question et un groupe de fibres indiciaires. Pour ce faire, il est possible d'utiliser une approche probabiliste, plus particulièrement une approche Bayésienne, afin d'assigner un rapport de vraisemblance aux observations issues de l'exploitation en laboratoire. Plusieurs paramètres doivent alors être discutés : la rareté des fibres observées (occurrence), la probabilité que ces dernières proviennent de l'action criminelle (transfert et persistance) et la probabilité qu'elles soient présentes par hasard/non liées au crime (bruit de fond). Le présent article propose une revue critique des sources d'informations publiées dans la littérature forensique permettant d'estimer les paramètres du rapport de vraisemblance. Les principaux résultats sont présentés puis discutés à la lumière des avantages et des limitations des différents types d’études. Dans cette première partie, seules les études dédiées à la détermination de la rareté des fibres indiciaires (occurrence) et aux fibres en provenance du bruit de fond seront présentées. Les travaux traitant du transfert, de la persistance et du prélèvement des fibres seront traités dans une seconde partie. English: The question at the centre of forensic expertise is the determination of the probative value of a correspondence between recovered trace evidence and suspect material. In the field of textile fibers, it is necessary to decide on the meaning of non-differentiation between a textile in question and a reference group (of control) fibres from a putative source. To do this, it is possible to use a probabilistic approach, in particular a Bayesian approach, in order to assign a likelihood ratio to the observations resulting from laboratory examinations. Several parameters have to be considered: the rarity of the observed fibres (occurrence), the probability that the latter will come from the criminal action (transfer and persistence) and the probability that they are present by chance (not related to the criminal activity). The present article is a critical review of the sources of information published in the forensic literature that can be used to estimate the parameters for the calculation of the likelihood ratio. The main results are presented and are discussed in the light of the advantages and limitations of different types of studies. In this first part, only the studies dedicated to the determination of the scarcity of the index fibres (occurrence) and the ‘background’ fibres will be presented. Work on the transfer, persistence and recovery of fibres will be dealt with in Part Two

Les paramètres clés de l'interprétation des fibres textiles en sciences criminelles. Partie II: Transfert, persistance et détection

Abstract Determining the probative value of trace evidence is essential in major criminal investigations. However, this can often be difficult when the significance of the results of a particular examination have to be evaluated within the context of the framework of circumstances of a specific case. This is particularly true in criminal investigations where it is necessary to evaluate the significance of a non-differentiation between fibres from a crime scene (or a crime-related item) with a source belonging to a suspect. By employing an approach involving a probabilistic Bayesian framework, it is possible to evaluate the significance of the results of an examination by considering a set of alternative propositions relating to the alleged activity in question. In such circumstances, it is crucial that any such evaluation is carried out at activity level, since transfer and persistence of textile material may be due to an activity not related to the alleged criminal action under consideration. This article reviews the literature on the detection, transfer and persistence of textile fibres since the understanding of these three concepts is essential for the robust assessment of activity level propositions. In addition, some complex casework scenarios are discussed, including secondary transfer phenomena and the transfer of fibres during domestic violence. Finally a section dedicated to the simulation of criminal and ‘legitimate’ activities involving fibre transfer is considered, which illustrates the importance of evaluating activity level propositions using data derived from conditions as close as possible to the framework of circumstances of the actual case under consideration. RÉsumÉ Déterminer la valeur probante d’un indice est au centre du travail de criminaliste car il peut être difficile d’apprécier pleinement la portée de certains résultats forensiques. Cela est notamment le cas des expertises impliquant des fibres textiles où il s’agit par exemple d’interpréter la signification d’une concordance entre un certain nombre de fibres retrouvées sur la scène de crime et un textile appartenant à un suspect. Suivant une approche probabiliste Bayésienne, il est possible de se prononcer sur la valeur probante des résultats en considérant un set de propositions alternatives et d’inférer sur l’activité à l’origine des traces. Il est vivement recommandé de discuter l’origine des traces au niveau de l’activité dans le cas des fibres textiles car ces dernières peuvent être échangées à l’occasion de nombreuses actions parfois sans lien avec les événements criminels investigués. Le présent article propose une revue de la littérature consacrée au transfert, à la persistance et à la détection des fibres textiles. Il s’agit de trois notions essentielles à l’évaluation des fibres au niveau de l’activité. Certaines situations complexes sont également discutées, notamment les phénomènes de transfert secondaire et les transferts de traces matérielles lors de violences domestiques. Finalement, une section dédiée à la réalisation de simulation d’actions criminelles et légitimes est proposée. Il s’agit d’une solution de choix, voire nécessaire, permettant d’étudier le transfert et la persistance des fibres dans des conditions se rapprochant au plus près du cas investigué