Forensics on trial: chemical matching of bullets comes under fire

Science News, March 27, 2004 by Alexandra Goho

In 1997, a jury convicted Michael Behm of murdering a man in South River, N.J. The only physical evidence linking Behm to the murder was bullet fragments from the crime scene. An FBI examiner testified in court that the fragments chemically matched bullets from a box of ammunition Behm had at his home. "We were devastated by this," says Jacquie Behm, whose brother is now serving a life sentence for murder. "At the time, we didn't know anything about bullet-lead analysis." Nor could her brother's lawyer during the trial find anyone qualified to question the validity of the chemical evidence or the examiner's interpretation of it.

As it turns out, there should have been plenty to question. Since that trial, a growing body of research has revealed that the practice of chemically matching bullets is seriously flawed. This February, a report released by the National Academies in Washington D.C. called on the FBI to revise its rules on interpreting data from chemical analyses of bullets and to limit how its examiners testify about such data in the courtroom.

Behm's present lawyer, Paul Casteleiro, has since filed a motion asking the courts to consider the National Acadamies' report in deciding whether to grant his client another trial. Other lawyers and their clients are likely to follow suit.

Indeed, the implications are considerable. The FBI has used chemical analysis of bullets in some 2,500 investigations since the early 1980s. Among those, there were 500 cases in which the prosecution introduced such analyses as evidence during trials. But the story of bullet chemical analysis has even broader implications; it emphasizes the need to keep science honest, especially in the courtroom.

MATCHMAKING For several years, statisticians, metallurgists, and others outside the FBI have questioned the courtroom use of bullet chemical analyses. Now, the National Acadamies' report, Forensic Analysis: Weighing Bullet Lead Evidence, has brought the practice into the spotlight. That report, combined with past studies detailing the forensic tool's shortcomings, could call into question many past convictions in which results of the chemical analysis of bullets was introduced as evidence.

First developed in the 1960s, bullet chemical analysis has been used by prosecutors when a suspect's weapon is not available or when the bullet found at a crime scene is too fragmented to permit visual inspection of the characteristic markings that firearms leave on intact bullets.

The chemical analysis consists of measuring seven trace elements--arsenic, antimony, tin, copper, bismuth, silver, and cadmium-that typically are present in a bullet's lead alloy. Each element makes up less than 1 percent of the total lead alloy.

Using a technique called inductively coupled plasma-optical-mission spectroscopy, a forensic chemist determines the proportion of each element in the lead alloy. In that analysis, the chemist dissolves a sample of the bullet and feeds the resulting solution into the instrument's plasma chamber, where each element in the sample emits specific wavelengths of light. The pattern of emissions serves as a fingerprint for that element, so the intensity of the light of each pattern indicates how much of the element is present.

Characterizing a bullet's chemical composition is relatively straightforward. What it all means, however, is a matter of interpretation. The traditional reasoning has been that if two bullets are chemically indistinguishable, they probably came from the same pot of molten lead at the smelter or were manufactured on the same day by the same company. In court testimonies, FBI examiners have gone so far as to say that two chemically indistinguishable bullets probably came from the same box of ammunition.

Several years ago, while still working at the FBI, metallurgist William Tobin began questioning this practice. After all, he notes, much was and still is unknown about bullet manufacturing. It is disingenuous to say that the matching of two bullets is a significant find without knowing how much chemical diversity there is in the general population of bullets, Tobin says. "There isn't an individual on the face of the earth qualified to interpret the forensic significance of bullet-lead analysis" he argues.

After retiring from the FBI in 2000, Tobin partnered with Erik Randich, a forensics consultant and metallurgist at Lawrence Livermore National Laboratory in California. The duo set out to examine whether there was any statistical basis to bullet-lead matching. The metallurgists contacted two lead smelters that supply ammunition manufacturers in the United States and pored over the smelters' production data.

These lead suppliers are called secondary smelters because the majority of their lead comes from spent automotive batteries rather than from ore. Most of the recycled lead goes back into making new batteries, so the refiners adjust trace elements in the lead to meet the specifications of the battery industry. Smelters keep detailed records on the elemental composition of the molten lead in each pot.