Comparative bullet-lead analysis, based on the assumption that all bullets in one batch will be chemically similar, examines the chemical compositions of bullets to determine if crime-scene bullets match bullets in a suspect’s possession. FBI labs have since concluded that all bullets in a single batch are not always chemically matched “because subtle chemical changes occurred throughout the manufacturing process.”FBI concerns over comparative bullet-lead analysis were first documented in 1991, and a study by two former FBI lab technicians challenging the technique was completed in 2001.
In 2004, the National Academy of Sciences also rejected comparative bullet-lead analysis, stating “that decades of FBI statements to jurors linking a particular bullet to those found in a suspect's gun or cartridge box were so overstated that such testimony should be considered ‘misleading under federal rules of evidence.’” A year later, FBI lab director Dwight Adams recommended to FBI Director Muehller that the Bureau abandon the comparative technique and discourage prosecutors from using it in future trials.
Adams believes that the government has an obligation to review cases in which the technique was used and to notify courts of any convictions that could have been erroneously based on the technique. "It troubles me that anyone would be in prison for any reason that wasn't justified. And that's why these reviews should be done in order to determine whether or not our testimony led to the conviction of a wrongly accused individual," Adams said to the Post. "I don't believe there's anything that we should be hiding."
The Post and 60 Minutes conducted a nationwide investigation, researching court files and holding interviews with dozens of lawyers and scientific experts. Their research yielded at least 250 cases in which evidence from comparative bullet-lead analysis was introduced. More than a dozen of these convictions have been reversed or are now being challenged as to whether innocent people were sent to prison. The FBI has said it would conduct a national review of these cases and create a system where future scientific testimony can be monitored.