First it was handwriting, hair and fiber comparison. Then it was polygraphs and fingerprints. Now it's the law enforcement technique of bullet lead analysis to come under legal and scientific attack.

For over 20 years, the FBI has been using chemical analysis of bullet lead to match bullets to the gun they were fired from and even to the ammunition box they came from. This technique, like the others mentioned, has now come under scientific attack. "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."

It seems that contrary to what FBI agents have testified to in court the past few decades, bullets that appear chemically identical can come from different batches of lead.

FBI experts have testified repeatedly in court that each time lead is melted and cast, its composition changes just enough to create a unique chemical signature for each batch.....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.

A separate study, commissioned by a former FBI agent and a metallurgist who once worked together on the Unabomber case, was conducted by four researchers.

...The four researchers found that there is no such signature. Trace elements are often distributed unevenly within each casting, they reported, so that entire batches rarely bear a uniform chemical profile. As molten lead is cooled, the trace elements tend to concentrate, clumping silver, arsenic or tin toward the center, the study found. As a result, the researchers said, bullets created from the same block of lead can appear unrelated and bullets produced years apart can appear virtually identical.

Defense counsel around the country have begun contesting the FBI's technique--and some are trying to set aside clients' convictions where the technique may have played a principal role at trial.

Richard Saferstein, former chief forensic scientist for the New Jersey State Police, said in a recent interview that the problem with bullet-lead analysis is not with the lab work but with the conclusions drawn from it. The FBI, he said, has tended to overstate the significance of such evidence. "It's the image that the FBI projects, of course, more than the technology. That image, when you are talking to jurors, is impressive," he said. "You can't abuse that privilege. It comes back to haunt the agency and the profession."