FBI, Defense Lawyers Review Bullet Lead Analysis Testimony
November 19, 2007 -- The FBI and criminal defense lawyer groups announced that they will review cases where the bureau used a forensic technique known as compositional analysis of bullet lead (CABL) to secure convictions in criminal cases, particularly murder prosecutions. The technique, which matches bullets from a crime scene to those in a suspect’s possession, was abandoned by the FBI after a 2004 National Research Council report concluded that the technique was flawed in part and that its limitations should be clearly conveyed in court.
Different from ballistics techniques that compare striations on the barrel of a gun to those on a recovered bullet, CABL is used when no gun is recovered, or when bullets are too small or mangled to observe striations. Since the 1960s, expert witnesses have used CABL evidence to testify that bullets found to be analytically indistinguishable probably come from the same “box” or “source.”
CABL has three distinct steps: first, chemical analysis of trace elements in the bullets’ lead ; second, statistical comparison of the lead compositions ; and third, the legal interpretation of data derived from the first two steps. The Research Council report Forensic Analysis: Weighing Bullet Lead Evidence found that the technique the FBI uses for chemical analysis is accurate and reliable but that the statistical tests used to say that two samples of bullets are indistinguishable should be improved.
Variations in the manufacturing process for bullets also sharply limit how CABL findings can be used in court. Every step from smelting the lead to buying the ammunition in a store provides opportunities for bullets with different compositions to be mixed and for bullets with the same composition to be shipped separately to different outlets in a region or to different regions. In fact, the FBI’s own research shows that a single box of ammunition can contain as many as 14 distinct compositional groups.
The report concludes that it is important that criminal justice and legal professionals, as well as juries, understand both the capabilities and limitations of this forensic technique.