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 infbnnation 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 ofknives 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 ofknives.

Chemical Analysis of Tire Traces in Traffic Accidents Investigation

The aim of the forensic investigation of traffic accidents is to help establish the nature and/or the circumstances of the event.This might be done with the purpose of determining the legal responsibilities of each person involved or to provide families,with a reconstruction of the events,to help understand why their relatives were injured or killed.A methodology for the comparison of chemical profiles of tire traces and tire tread samples obtained by pyrolysis-gas chromatography/mass spectrometry has been developed.Chemical profiles are represented by relative abundances of 86 compounds.The variability of the tread within and between 12 tires was assessed.Considering the level of the source as"brand and model"the intra-variability was found to be smaller than the inter-variability leading to the complete discrimination of the 12 tires of the sample set.Braking tests were carried out on a racetrack in order to produce tire traces which origin was known.The results obtained with a supervised classification method showed that more than 94%of the replicates of the traces were correctly assigned to the class membership(i.e.,brand and model)of the tire at their origin.These results support that the chemical profile of one trace does not differ from the chemical profile of the tire at its origin but differs generally from the other chemical profiles of the sample set.