This paper examines the application and subsequent invalidation of comparative bullet lead analysis (CBLA), a forensic technique used by the FBI in approximately 2,500 criminal prosecutions since the 1960s. The paper outlines the scientific method's role in forensic investigations, explains the hypothesis underlying CBLA, and traces the 1998–2001 study by William Tobin and Lawrence Livermore National Laboratory researchers that disproved the technique's core assumptions. It concludes by discussing the FBI's formal response, the limits of expert testimony, and what role, if any, CBLA may play in future criminal proceedings.
The scientific method generally consists of forming and testing hypotheses to explain an observed phenomenon or group of phenomena. More specifically, scientific inquiry consists of disprovable hypotheses capable of being tested in relation to corresponding phenomena or quantitative predictions in ways that can either establish a logical basis supporting the proposed explanation or refute it. Scientific tests must also be repeatable by other experimenters. In criminal investigations, the scientific method underlies every component of forensic analysis. Furthermore, aside from the requirement that any method used to establish evidence of a crime be supported by a valid, testable, and repeatable hypothesis, the burden of proof on the prosecution also requires that any relevant test used to establish guilt be sufficiently accurate to a very high degree of certainty (Kobalinsky and Liotti, et al., 2005).
One of the most dramatic examples of the use of the scientific method in forensic science involved the testing of the validity of a sophisticated modern forensic technique used to establish guilt in hundreds of criminal cases since the 1960s. The results of those tests ultimately led to the invalidation of comparative bullet lead analysis (CBLA), a method used extensively by the Federal Bureau of Investigation (FBI) to establish the identity of criminal defendants accused of crimes involving firearms. Comparative bullet lead analysis was first used in 1963 in connection with the assassination of President John F. Kennedy (Solomon, 2007). In principle, the technique is predicated upon the assumption that bullets manufactured within the same batch bear statistically relevant similarity that distinguishes them from bullets produced in other batches.
In theory, the measurable differences in chemical composition within bullets could allow the technique to positively identify fragments recovered from crime scenes and link them definitively to other bullets recovered from suspects. This method proved extremely useful to law enforcement investigations, particularly in situations where insufficient evidence was available on bullet fragments for gun-barrel groove marking analysis. The entire prospect of using comparative bullet lead analysis in criminal prosecution was successfully challenged on scientific grounds in 2002, and internal FBI documents disclosed during that investigation suggested that the bureau had expressed similar concerns for more than a decade (Solomon, 2007).
The hypothesis behind bullet lead analysis is straightforward: specific elements in the lead of bullet fragments vary consistently between different batches of lead used in their manufacture, and those differences can be measured precisely enough to exclude bullets from having come from the same box of ammunition through chemical analysis of their specific lead content. The hypothesis arose from the fact that American bullet manufacturers typically use large quantities of lead obtained by melting down recycled car batteries. The expectation was that large batches of lead used by each manufacturer to produce ammunition within boxes made at approximately the same time would yield individual bullets of much closer chemical composition than boxes manufactured from different batches of lead (Solomon, 2007).
This hypothesis is analogous to the methods used to establish paternity using DNA analysis. Instead of genetic information extracted from biological tissues, comparative lead analysis relies on the measurable differences of various trace chemical elements, such as antimony, arsenic, and copper (Solomon, 2007).
In its initial incarnation, the differential chemical analysis was conducted by directing neutron beams at bullet fragments. In 1996, the neutron beam method was replaced by a form of optical spectrum emission spectroscopy that the bureau calls inductively coupled plasma optical emission spectroscopy. The principal benefit of the new method was that it provided measurements of four additional trace elements — bismuth, cadmium, tin, and silver — beyond the original three. One of the expected advantages of using seven markers instead of three was increased precision in identifying bullet origin (Solomon, 2007).
"Scope of FBI CBLA use in U.S. prosecutions"
"Tobin study disproves CBLA's foundational assumptions"
In the case of comparative bullet lead analysis, the underlying hypothesis — that the relative chemical composition of bullets varied measurably on the basis of different batches of lead used in their manufacture — was actually valid, in principle. However, more careful investigation disclosed two critical problems: (1) the degree of variation was too inconsistent to be relied upon to the extent suggested by FBI testimony, and (2) bullets bearing chemically distinct similarity could not be positively identified as having come from the same box of ammunition, but only as having come from the same general community (Solomon, 2007).
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