Forensics on trial: chemical matching of bullets comes under fire

Science News, March 27, 2004 by Alexandra Goho

When Tobin and Randich looked at the composition records for different pots of molten lead, they saw reason for concern. The composition of castings from a single pot sometimes varied, while the composition of lead in different pots sometimes matched. That meant that bullets made from two different batches of lead could wrongly appear to have come from the same pot.

"We then knew that both of the assumptions that the FBI makes--that a lead source is homogeneous and unique--are not true," says Randich.

IT'S CIRCUMSTANTIAL

With the publication of Tobin and Randich's research in July 2002, as well as other studies including the FBI's own analyses, pressure mounted on the FBI to reevaluate its methods and court testimonies. In the fall of that year, the bureau asked the National Acadamies to put together a committee to formally review the FBI's use of bullet-lead analysis and recommend changes.

The uncertainty of lead's provenance doesn't end with the smelting process, says Kenneth MacFadden, an independent consultant with training as an analytical chemist, who chaired the National Acadamies' committee. "Bullets from one [lead source] can get mixed with bullets from another at various points in the manufacturing process" he explains.

Once a bullet manufacturer receives slabs of lead from a refiner, the lead is cut into smaller blocks called billets. The billets are extruded into wires, which are cut into slugs that are then pressed into bullets. Because manufacturers receive lead from different smelter pots, lead from different sources can be intermingled at many stages in the manufacturing process. Therefore, a box of ammunition is likely to contain bullets from multiple volumes of lead, the committee reported.

"In fact, the FBI's own research has found instances where a single box of ammunition contained as many as 14 distinct compositional groups," MacFadden says.

The committee concluded that it's impossible to determine that a bullet from a crime scene came from a particular box of bullets or that two bullets were manufactured on the same day at the same factory.

This finding greatly weakens the evidentiary value of bullet-lead analysis. The committee recommended setting narrow limits on what FBI examiners can say in court. For instance, should two bullets have matching compositions, instead of suggesting they came from the same box of ammunition, an FBI expert can merely testify to an increased probability that the two bullets came from what the committee has called the same "compositionally indistinguishable volume of lead" (CIVL).

Acknowledging that smelting pots come in different sizes and that the chemical makeup of lead can vary within a pot, the committee asked that FBI examiners avoid making reference to "melt" or "production run." MacFadden adds that experts should explain to jurors that a CIVL can be of different sizes and produce anywhere from 12,000 to 35 million .22-caliber bullets. Annually, 9 billion bullets are made in the United States.

Committee member Paul Giannelli of Case Western Reserve University's Law School in Cleveland, likens attempts to match the lead from different bullets to finding a Nike size-10 shoe-print that matches that of a suspect's size-10 Nikes. "It's only circumstantial evidence," says Giannelli. "It would be admissible in court, although there would be a large number of people with that type of shoe." Similarly, if forensic analysis showed that the composition of a bullet from a crime scene matched that of a bullet confiscated from a defendant, there still would be many other people in possession of matching bullets.