FBI reviews cases where flawed evidence was used but it still hasn’t completed the review. Five years after the decision to drop the disputed comparative bullet lead analysis, the FBI has not completed the review of almost 2500 cases. In all those cases police used the disputed comparative bullet lead analysis.
“So far, the agency has found 187 cases where so-called comparative bullet lead analysis evidence was not only used in the investigation, but came into play at trial where FBI experts provided testimony. It has notified prosecutors in those cases where testimony from its experts “exceeds the limits of the science and cannot be supported by the FBI,” one agency letter says.”
Comparative bullet lead analysis uses chemistry to link bullets found at a crime scene to bullets found in the possession of suspects. The theory was that each batch of lead in those bullets had a unique elemental makeup.
Lead bullets pick up trace elements such as copper, antimony, arsenic, bismuth and silver during manufacturing. When the soft metal is shaped into bullets and packaged, bullets in the same box would contain similar amounts of the trace elements. So FBI lab technicians compared bullet fragments from a crime scene with bullets found on suspects. If the trace elements closely matched, prosecutors – backed by FBI testimony – would argue the suspects’ guilt. This is what was done since the mid-60s.
In 2005, the National Academy of Science concluded that while the method for measuring trace elements is sound, the conclusions drawn were not. Millions of other bullets could contain trace elements in similar quantities. Read more here.