Can We Really Say Where a Particular Bullet Came From?

For decades, FBI examiners operated on the twin assumptions that every batch of bullet lead was compositionally uniform throughout and that no two batches were compositionally alike. So if two bullets were found to have the same concentrations of the same elements, the reasoning went, those bullets must have come from the same batch of lead.

In the past few years, however, critics have begun to question those assumptions. Some researchers have found that different samples from the same batch of bullet lead can have different elemental compositions. Other research has confirmed that different batches of bullet lead can have the same elemental composition. Still other research has shown that bullets from the same box of ammunition can have different elemental compositions, some of which can be attributed to bullets from different melts and some of which can be attributed to intramelt variability.

?This is not a case in which the critics of a forensic practice are merely pointing to a lack of research data to validate an assumption,? says University of California at Davis law school professor Edward J. Imwinkelried, who co-wrote a critical law review article about bullet lead evidence last year. ?This is an even more troublesome situation in which there are available data at odds with both of those assumptions.?

It sickens me that people have been jailed or executed on this kind of testimony. The problem with the FBI crime labs is that they don't have to turn up any actual evidence; they just have to look convincing in front of a jury. Once expert testimony has been accepted in a few cases, it becomes a self-fulfilling prophecy: people are convicted based on that evidence because other people were convicted based on that evidence. And yet, we know from such things as the BATFE testimony ("we testify in court that the database is 100% accurate, even though we know it isn't so") that the standards really more are appropriate to government work than a life-or-death matter for the accused.