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FBI bullet-matching tests under fire

In a finding that could affect thousands of cases, the National Academy of Sciences has concluded that some techniques the FBI has used for decades to match bullets to crimes are flawed or imprecise.

In a finding that could affect thousands of criminal cases, the National Academy of Sciences has concluded that some techniques the FBI has used for decades to match bullets to crimes are flawed or imprecise.

THE STUDY, expected to be released in the next few weeks, makes about a half-dozen recommendations to improve the FBI lab’s science used to match bullets through their lead content.

The academy’s findings, which are in final draft form, were described to the Associated Press by several people involved in the study. They would speak only on condition of anonymity.

The study specifically urges the bureau’s chemists to stop a practice known as data chaining that chemists have used in the past to match bullets to a crime.

In data chaining, scientists can conclude that if the lead content of bullet A matches bullet B, and bullet B’s content matches bullet C, then it is safe to testify that bullet A and bullet C are a match even if their test results don’t match identically. Put another way, the FBI can match two dissimilar bullets if they can find a third — from a manufacturer, for instance — that comes close to matching both.


The FBI science relies on the theory that bullets from the same batch of lead share a common chemical fingerprint.

Charles Peters, an FBI’s expert witness in cases involving bullet lead comparison, testified recently that data chaining — the technique disavowed by the academy — was important to matching bullets.

“I’m a fan of chaining,” Peters testified in April in a case in Alaska. “If ... we didn’t do something like chaining, or something like that, nothing would ever match.”

A reference in the latest draft of the academy report indicates the FBI may abandon the data-chaining technique, the sources said. FBI officials said Thursday night they had not seen the report and could not comment on it.

National Academy of Sciences spokesman Bill Kearney also said he could not comment on a “draft report that is still being peer reviewed and subject to change.”

Citing specific examples of conflicting or inconsistent testimony by FBI experts, the study also recommends that lab analysts’ work and testimony be reviewed by a peer to ensure accuracy and precision, the sources said.

The FBI lab’s director has been trying to increase the number of peer reviews inside the lab.


The academy’s recommendations are likely to have a huge impact, opening the door for appeals from defendants convicted in past cases where bullets were matched by the FBI using lead analysis. It also could force FBI lab witnesses to more narrowly describe the statistical significance of their findings in future cases.

The FBI has been the prime practitioner of lead bullet comparisons in the United States, and has used it for decades, dating to around the time of President Kennedy’s assassination 40 years ago. A database of lead test results kept by the agency had more than 13,000 samples in the late 1990s, FBI officials have told the AP.

The FBI most commonly identifies bullets recovered from a crime by firing new bullets from the suspect’s weapon and comparing the markings left by the gun barrel on the test bullet with the crime scene bullet. But that method only works when the crime scene bullet is in good shape or if police have the suspect weapon.

In cases where recovered crime scene bullets are fragmented or disfigured or a suspect’s weapon is unavailable, the FBI has turned to chemical analysis to try to determine whether the bullet’s lead content is comparable to the same manufacturer, lead source or box of bullets connected to the suspect.

When the lab makes a match, its experts testify that two bullets are “analytically indistinguishable.”

FBI Lab Director Dwight Adams earlier this year asked the academy to review the lead bullet identification process after one of the bureau’s most respected metallurgists, after he retired, began openly challenging his former employer’s science. The FBI paid for the study by the academy, which is one of the nation’s premiere scientific institutions.


The former FBI metallurgist, William Tobin, and his colleagues have published research stating that bullets from the same lead source had different chemical makeups and bullets from different lead sources appeared chemically similar, challenging the very premise of the FBI’s science.

Testifying as a defense expert, Tobin has cited evidence that FBI lab experts have testified in conflicting manners about how lead composition can identify bullets and link them to criminals.

Iowa State University has conducted research that drew similar conclusions.

“The fact that two bullets have similar chemical composition may not necessarily mean that both have the same origin. ... The leap from a match to equal origin is enormous and not justified given the available information about bullet lead evidence,” Iowa State researchers reported.

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