Application of lead isotope signature and likelihood ratio evaluation in a shooting incident investigation

Bullet projectiles are often found in shooting crime scenes and can help identify the cartridges used in shooting events. The most conclusive way to link a fired projectile to a firearm is by identifying the unique markings of the barrel on the projectile or case. But in circumstances where the investigation of striation marks is impossible, lead isotope ratio signature analysis of projectile fragments can facilitate building the linkage of the suspect, the cartridge used and the victim. This work presents a shooting incident happened in a hunting field, where a hunting guide was shot to death. A tiny lead projectile fragment was collected from the victim's head. Pb isotope ratios measured by laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) were used to make comparisons between the fragment and the shots seized from two suspects. Additionally, a reference dataset comprising 50 lead bullet projectiles from various manufacturers was characterized using LA-MC-ICPMS. The evaluation of the comparison was carried out by multivariate likelihood ratio (LR) computations based on the reference dataset. The lead isotope ratio and likelihood ratio evaluation methods helped infer the source of the fragment and were highly supportive in the shooting incident investigation.