The assumption that bullets found at a crime scene can be matched to those in a suspect's possession has helped convict countless murderers, robbers and armed felons in the US, Britain and elsewhere. Forensic scientists analyse lead bullets for traces of antimony, tin, arsenic, copper, bismuth, silver and cadmium. The idea is that if two bullets have the same chemical signatures, they must have been made at the same time from the same batch of smelted lead. British firearms expert Jonathan Spencer, from the forensic services firm Keith Borer Consultants in Durham, says it is also common for this link to be made in court cases in Britain.
But New Scientist has learned that this assumption is plain wrong. An examination of detailed records held by manufacturers of the lead used to make ammunition shows it is impossible to prove that any two bullets come from the same batch.
"If you're in a court of law, you really need to do your homework, because you're putting people in jail or on death row," says Erik Randich, the forensics consultant and metallurgist based at Lawrence Livermore National Laboratory who made the discovery. No one knows how many verdicts have rested on such evidence, and it is unclear whether the revelation would have changed the verdict against Behn. But there are now fears that the technique may have directly or indirectly led to numerous miscarriages of justice.
Randich says one FBI analyst told him that he has testified in hundreds of court cases that bullets could be matched in this way. Randich has informed the FBI of his findings, but the bureau did not answer New Scientist's requests for comment on whether it plans to change its procedures.
No one is questioning whether the rifling on a bullet can be matched to a specific gun barrel or other spent bullets, but whether a chemical analysis of a slug can pinpoint its source. Until now, the chemical bullet-matching technique had never been double-checked. So Randich and former FBI chief metallurgist Bill Tobin analysed several years of data kept by two lead smelters that supply the raw material for ammunition.
Smelters carefully monitor the levels of the trace elements in their lead in order to meet standards set by the battery industry, which uses most of the lead produced in the US. Lead for ammunition comes from the same batches. The manufacturers record the levels of at least 15 trace metals, including those measured by the FBI, from samples taken when smelters start and finish pouring a batch of lead into casts.
The researchers examined records for 1998 to 2000 held by Sanders Lead Company in Alabama and Gopher Resources Corporation in Minnesota. They found many instances where it was impossible, using the FBI's chemical profile standards, to distinguish between batches poured months apart. Another analysis of an earlier set of Sanders records revealed the same problem.
The researchers also found small but measurable differences in the composition of lead samples taken at the beginning and end of the same batch, probably due to oxidation of the trace elements. That means it's impossible to say whether any two bullets were made on the same day or come from the same box.
"You'll find bullets that are indistinguishable that were made months apart," says Randich, who presented his results last week to a meeting of the American Chemical Society in Orlando. Randich and Tobin, along with analytical chemist Wayne Duerfeldt at Gopher Resources and metallurgist Wade McLendon at Sanders, will also publish their findings in a future issue of the journal Forensic Science International.
Tobin was never involved in the forensic testing of lead ammunition during his time at the FBI. But he became so concerned by the interpretation of the bullet data that he contacted Randich once he had retired. "It offended me as a scientist," he says.
In 1999, the FBI commissioned its own research into the validity of bullet matching. The bureau asked Alicia Carriquiry, a statistician at Iowa State University, to determine whether it was possible to give a statistical probability that two bullets came from the same source. Her study, which she says she submitted to the FBI in May 2000, found that while it is theoretically possible to determine the likelihood of a match, the FBI didn't have enough data to do it.
Carriquiry says Randich's work shows there is a high probability that a match is simply due to coincidence. Spencer agrees: "It's taken as pretty strong evidence. But it should be wrapped in a heavy caveat, and I've never seen that done in court."
Authors: Robin Mejia and Ian Sample
New Scientist issue: 20 April 2002
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