The body of coin dealer Robert Rose was discovered in his Main Street office in South River, N.J., on a steamy July evening in 1995. He had been shot four times in the head.
There were no witnesses, no fingerprints, no gun.
But a chemical analysis of bullets by the FBI seemed to conclusively link the rounds that killed Rose to a box of cartridges belonging to one of his customers, Michael Behn. Lead in the different bullets bore the same telltale pattern of impurities, an FBI expert told the jury. Behn was convicted of the murder.
The same technique has been used in thousands of criminal cases over the last 30 years. Testimony by FBI experts about chemical "matches" between bullets has helped put hundreds of defendants behind bars across the country. In one Texas case, such testimony contributed to the conviction of an accused murderer, who was put to death.
Now, emerging scientific evidence has called the technique into question.
A Times examination of technical studies and trial transcripts -- and interviews with former FBI technicians, independent scientists and legal scholars -- suggests that the bureau's use of evidence derived from the lead in bullets may be based on faulty assumptions that greatly overstate the importance of matches.
The FBI likens its lead technique to fingerprint analysis. Bullets found at crime scenes are tested for minute amounts of arsenic, tin, silver and other contaminants or additives. Those findings are compared with results of similar analysis of bullets found in the possession of suspects. FBI examiners have claimed in court to be able to link one bullet to others from the same production run -- even from the same box.
The technique has proved especially important in cases in which prosecutors have little or no direct evidence, such as fingerprints or an eyewitness identification.
There is no dispute that trace elements of chemicals can be precisely measured in bullets. The controversy centers on how the FBI interprets the data.
For years, FBI laboratory examiners operated on the assumption that each batch of bullet lead was unique. So if the same trace elements were found in the same concentrations in two bullets, the reasoning went, those bullets must have been made at the same time and in the same place.
Recent scientific studies have concluded that this premise is wrong. Studying blocks of lead used in the manufacture of bullets, researchers have found the same chemical makeup in batches made at different times. They also have reported that the concentration of trace elements can vary significantly in the same casting of lead.
If the skeptics are right, the matches found by FBI lab technicians are meaningless.
"The many technical flaws in the FBI's bullet-lead analysis could well call into question hundreds of convictions," said William Tobin, a metallurgist, former FBI crime lab examiner and coauthor of one of the recent studies.
FBI officials who supervise bullet analysis and the examiners who carry it out declined repeated requests for interviews.
In a statement, the bureau said: "We've been employing these methods and techniques for over 20 years in the FBI crime lab. They've been routinely subjected to vigorous defense scrutiny in the courts and we feel very confident that all of our methods are fully supported by scientific data."
Nevertheless, individual FBI experts have begun to acknowledge that the technique is not as accurate as bureau officials have long maintained.
In a recent technical publication, and in court testimony last fall, FBI lab professionals said the bureau's research showed that bullets from different batches of lead could be chemically indistinguishable, and that lead from the same source could vary in its composition.
Further evidence of deficiencies in the FBI method came from a statistical study commissioned by the bureau itself.
A team of Iowa State University statisticians studied FBI data on 800 bullets. They concluded that even in that small sample, many chemical matches between bullets could be the result of chance.
The researchers said that a much more extensive study would be needed to determine whether accidental matches are so common as to render the FBI method useless.
The new information has stirred the beginnings of a legal assault on the once-undisputed forensic technique. Some defense lawyers are hiring experts to challenge FBI testimony about analysis of bullet lead. In some recent criminal trials, they have won acquittals. Other lawyers are seeking to overturn old convictions based on bullet analysis.
A variety of forensic techniques, including polygraph tests, fiber analysis and fingerprinting, have come under legal challenge in recent years. The problem, experts say, is that such methods evolved from hunches, trial and error, and anecdotal evidence rather than from accepted scientific practice, which requires controlled experiments and rigorous peer review of the results.