THE FBI finds a bullet at a crime scene, and matches its chemical composition to a batch of bullets found at a suspect鈥檚 house. It鈥檚 a straightforward bang-to-rights job, helping to secure a conviction.
Except that this assumption is flawed, and some convictions using this most cherished forensic technique might prove unsafe, according to a panel of scientists from the US National Research Council. On Tuesday, the panel released a report detailing a number of doubts about the reliability of 鈥渃ompositional analysis of bullet lead鈥�, echoing concerns highlighted by New 杏吧原创 two years ago (20 April 2002, p 4).
Developed by the FBI in the 1960s, the technique assumes that if two bullets share an identical chemical signature of contaminants such as antimony, tin, copper and bismuth in their lead, then they were made from the same batch. Not necessarily so, says the NRC report. Batches of melted lead may not always contain uniform profiles of such contaminants, and bullets from identical batches could be sold in different boxes. Most importantly, there is even the possibility that different batches could match entirely by chance.
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The panel concludes that it is insupportable to say that 鈥渁 crime bullet came from a particular box of ammunition鈥�, and references to 鈥渂oxes鈥� of ammunition could be misleading. Nor is it possible to use the technique to say when or where a bullet was made, as 鈥渄etailed patterns of the distribution of ammunition are unknown鈥�.