LEAD BULLET ANALYSIS
The theory behind lead bullet analysis was that bullets coming from the same box of ammunition would be coming from the same factory and would be manufactured at the same time, under similar conditions, and with the same lead composition present; all the bullets in a box, therefore, would have the same chemical makeup. Bullets from different boxes would have lead compositions. The theory was that if a bullet from the crime scene had the same lead composition as a bullet from a box of bullets owned by a suspect, there was physical evidence linking the suspect to the crime scene.
The conclusion of the National Academy of Sciences, after a thorough review of the state of the research, was that there was is no evidence to support the theories behind lead bullet analysis. Bullets from the same box may not necessarily have the same chemical makeup. Furthermore, bullets from different boxes – and different factories – may have the same chemical makeup. In addition, the FBI lab was overstating bullet matches even though there were differences in the measurements. This makes lead bullet analysis extremely unreliable, despite having been used for 40 years to convict many people. Analysis by the FBI lab was used as prosecution evidence in almost 2,500 cases.
In 2007, the Washington Post and CBS’ 60 Minutes collaborated on an investigative report examining the FBI’s use of the bullet lead analysis and their decision to stop using that method. The report found that the FBI first became concerned about the science after conducting a study in 1991. While the study supported the accuracy and reliability of the science, two warning signs stood out. First, the exact same lead composition was found in boxes of bullets manufactured months apart. Second, bullets in the same box often had different lead compositions. This meant that each box of bullets did not necessarily have a unique lead composition. The FBI now descredits any testimony where the analyst testified that all matching bullets had to have come from the same box.
In addition, the Post reported that a former FBI scientist began challenging the science. After retiring, the scientist conducted his own tests and found the science to be unreliable. He then began testifying as a defense expert where FBI bullet matching theory was being used.
The unfortunate consequence of unreliable science is its use to wrongfully convict innocent persons. During its review, the FBI uncovered several cases where convictions were tainted by expert testimony that lead analysis connected the defendant to the crime.
The history of bullet lead analysis shows how science must continually be the subject of research studies as to its reliability and accuracy. This is only way to prevent wrongful convictions from continuing.