Paul Gianelli’s Comparative Bullet Lead Analysis: A Retrospective (click on One-Click Download to read the full text article) which was published this month explains how a forensic technique was discredited and describes law enforcement’s insufficient response to the technique’s debunking.
Comparative Bullet Lead Analysis (CBLA) was a technique used for over 30 years in thousands of cases to compare trace chemicals found in bullets in an attempt to link bullets found at a crime scene with a particular batch of lead. The technique was used by FBI experts until 2005 and was also used in state prosecutions.
Challenges to the technique began to be mounted in 2002 and 2003, when retired FBI examiner William Tobin published scientific and legal articles questioning the procedure. Following Tobin’s articles and testimony in cases, the FBI asked the National Academy of Sciences (NAS) to review the technique.
Gianelli critiques the FBI’s delay in providing the NAS Committee with data needed to evaluate the technique. Though the data was provided too late for the Committee to use, the NAS Committee was able to determine that FBI experts were providing inconsistent interpretive conclusions. The 2004 NAS Report, Weighing Bullet Lead Evidence concluded that “[t]he available data do not support any statement that a crime bullet came from a particular box of ammunition. In particular, references to ‘boxes’ of ammunition in any form should be avoided as misleading under Federal Rule of Evidence 403.” (p.5)
Despite this finding, the FBI continued using the technique for another year. Gianelli concludes that the FBI’s response to the NAS Report and its press release when it discontinued use of the technique one year later minimized the problems. Gianelli explains that the FBI continued to supply affidavits supporting prosecution efforts to sustain convictions based on the technique after the report was published and declined to disclose the names of cases in which its experts had testified based on the CBLA technique.
Gianelli reports that the FBI laboratory director at the time, Dwight Adams, wrote a memo stating “[w]e cannot afford to be misleading to a jury” and “[w]e plan to discourage prosecutors from using our previous results in future cases.” (p. 8 ) However, the FBI’s press release said, “[w]hile the FBI Laboratory still firmly supports the scientific foundation of bullet lead analysis, given the costs of maintaining the equipment, the resources necessary to do the examination, and its relative probative value, the FBI Laboratory has decided that it will no longer conduct this exam.” (p. 8 )
In addition to demonstrating institutional resistance to abandoning a technique that has been disproved, Gianelli identifies several lessons learned from this process. He signals the need for empirical data that supports scientific conclusions and shows the difficulty for a single defense expert in a particular case to produce the research that is needed to critique a forensic technique.