FBI Forensic Evidence Discredited

Hugh Pickens writes: "The Washington Post reports that the FBI has abandoned comparative bullet-lead analysis, the technique using chemistry to link crime-scene bullets to ones possessed by suspects on the theory that each batch of lead had a unique elemental makeup, after the National Academy of Sciences said that decades of FBI statements to jurors linking a particular bullet to those found in a suspect's gun or cartridge box were so overstated that such testimony should be considered "misleading under federal rules of evidence." The report added that it found that bullets packaged 15 months apart — a span that assumed separate batches of lead — had the exact composition, potentially undercutting the theory that each batch was unique and that it found that bullets in a single box often had several different lead compositions. NAS says that the flaw is in using a statistical method called chaining (pdf) in which the analyst sequentially compares crime scene bullets to a set of reference bullets assembling them into groups of compositionally indistinguishable bullets which can lead to the formation of artificially large sets of matching bullets. The government has fought releasing the list of the estimated 2,500 cases over three decades in which the FBI performed the flawed analysis."