Abstract
The probative value of comparative bullet lead analysis (CBLA), a now discontinued technique that was used by the Federal Bureau of Investigation for more than 30 years, has been hotly debated over the last several years. One issue that has received relatively little attention concerns the degree of geographic dispersion of bullets as they pass from manufacturers to retailers. Proponents and critics of CBLA alike agree that geographic distribution is such a major consideration, if not a predominant one, that it could significantly diminish, or completely erode, the probative value of a CBLA 'match' or, in some cases, even make a match counter-probative. The inattention to this issue to date appears to be a consequence of lack of data, rather than lack of importance. Until now, no datum concerning bullet distribution has been presented in the public domain, critically hampering the proper estimation of the probative value of a CBLA match. In this paper, we use manufacturer packing codes on boxes of bullets in retail outlets at four sites in the United States as a surrogate measure of bullet lead compositions to gauge local retail bullet distribution. Using a weighted average packing code match probability, we found very high degrees of geographic concentration of bullet packing codes. Although these findings can only offer a rough estimate of the degree of geographic concentration of actual chemical compositions of bullets, they are sufficient to establish that geographic concentration does, in fact, exist. Such a concentration would have a significant impact on the probative value of any claimed CBLA match.
Highlights
Comparative bullet lead analysis (CBLA) is a forensic service that has been offered by the U.S Federal Bureau of Investigation (FBI) in criminal investigations since the 1960s
On 1 September 2005, the FBI declared the moratorium permanent indicating that ‘neither scientists nor bullet manufacturers’ could explain the forensic significance of a claimed match: One factor significantly influenced the Laboratory’s decision to no longer conduct the examination of bullet lead: neither scientists nor bullet manufacturers are able to definitively attest to the significance of an association made between bullets in the course of a bullet lead examination. (Federal Bureau of Investigation, 2005)
The fundamental premise of CBLA is that similarity of the chemical signatures between the crime-scene fragments and the bullets in possession of the suspect purportedly provides evidence that the bullets are from the same ‘molten source’, pour or lot of bullets made by the same manufacturer on the same day
Summary
Comparative bullet lead analysis (CBLA) is a forensic service that has been offered by the U.S Federal Bureau of Investigation (FBI) in criminal investigations since the 1960s. For most of that period, the relatively esoteric technique attracted little attention, in recent years the technique generated controversy as research into the practice expanded and a number of studies critical of CBLA were published (Imwinkelried & Tobin, 2003; Randich et al, 2002; Tobin, 2004; Tobin & Duerfeldt, 2002). These critiques were followed by successful legal challenges to the admissibility of CBLA under the Daubert standard for scientific evidence and by press coverage of the controversy (Hansen, 2004; Mejia & Sample, 2002; Piller & Mejia, 2003). The fundamental premise of CBLA is that similarity of the chemical signatures between the crime-scene fragments and the bullets in possession of the suspect purportedly provides evidence that the bullets are from the same ‘molten source’, pour or lot of bullets made by the same manufacturer on the same day. This, in turn, indicated that the crime-scene bullet may have come from the suspect’s cache of bullets
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