The idea that ballistics -- marks made on bullets and casings by the gun they are expelled from -- can be relied upon as evidence is something most of us are familiar with. We've seen it on the CSI shows.
In order to be useful evidence, a ballistics test would need to show with great certainty that a bullet from a crime scene had to have come from a particular gun. It shouldn't serve to convict anyone unless it conveyed some certainty backed by science.
Unfortunately, that scientific certainty is somewhat of an illusion when it comes to ballistics, according to a recent report by NPR.
In 2013, the issue came up in a death row case. The FBI sent a note to the prosecutor clarifying its position on ballistics testimony. It said that "the science regarding firearms examinations does not permit examiner testimony that a specific gun fired a specific bullet to the exclusion of all other guns in the world," and that claiming such certainty is "not supported by scientific standards."
"The problem is, no one's gone out and actually determined that it could only be matched to that gun to the exclusion of all other guns in the universe," explains the founder of the nonprofit Forensics Justice Project.
Indeed, a 2009 National Academy of Sciences report urged caution when using ballistics evidence. The academy found that insufficient study had been done to back up ballistics as reliable.
Nevertheless, the U.S. Department of Justice has been issuing grants to give local police departments -- including Des Moines -- access to the National Integrated Ballistics Information Network (NIBIN), a database of high-resolution images of bullets and shell casings. The database, which is administered by the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, is intended to allow police forces to match bullets and shell casings from one crime scene to another and, in some cases, match the bullet or casing to a specific firearm.
NIBIN matches may be being used primarily as investigative tools meant to provide leads. That would seem reasonable enough, but there are real issues with using it that way. Obtaining what seems like a match can create a cognitive bias in investigators, according to the Forensics Justice Project's founder.
"It's the idea that once we start building that narrative and it starts making sense the more things we see that fit into that same narrative," she explains.
We urge police and prosecutors to use ballistics responsibly, and we recommend the public take ballistics reports with a grain of salt, at least until further scientific research has been done.