A fired bullet (left) and bullet and cartridge case. Forensic experts can often link a bullet or cartridge case to the gun that fired it based on microscopic marks left on the bullet as it exits the barrel, or on the cartridge case as it is ejected from the firearm. Credit: Robert M. Thompson/NIST
The Organization of Scientific Area Committees (OSAC) for Forensic Science has placed two new standards covering firearm and toolmark analysis, the forensic discipline used in the investigation of gun crimes, on its Registry of Approved Standards. This stamp of approval from OSAC, which is administered by the National Institute of Standards and Technology (NIST), indicates that these standards are technically sound and will help forensic laboratories improve their processes and methods.
These are the 100th and 101st standards to be placed on the registry by OSAC, which works to strengthen forensic science in the U.S. by facilitating the development and adoption of high-quality standards. When the organization launched in 2014, there was a patchwork of standards but no coordinated effort to develop and improve standards across multiple forensic science disciplines.
Forensic experts can often link a bullet or cartridge case to the gun that fired it based on microscopic marks left on the bullet as it exits the barrel, or on the cartridge case as it is ejected from the firearm. For instance, to test whether a bullet found at a crime scene was fired from a specific gun, experts test-fire that gun to produce a second bullet. They then compare the microscopic marks on the pair of bullets to see how similar they are.
Examiners usually compare the bullets under a split screen comparison microscope, rotating and moving them to see if the markings line up — a method that has been in use for more than 100 years. However, researchers have been developing new methods that use 3D surface scanning microscopes to produce 3D models, or virtual copies, of the bullets. Computer algorithms then compare the microscopic features of the two virtual bullets to measure how similar they are.
3D methods are not yet widely used in forensic labs. But for labs that wish to implement them, the new standards provide guidance. The new standards are:
“These standards give labs guidance on purchasing and setting up a 3D system, validating it to ensure that it produces accurate results, and implementing it into their workflow,” said Erica Lawton, a firearms examiner at the Alabama Department of Forensic Sciences who, as the chair of OSAC’s Firearms and Toolmarks Subcommittee, helped guide the new standards through the approval process.
One of the benefits of a 3D system is that, after comparing the surface features of two bullets or cartridge cases, the algorithm generates a numerical score that describes how closely the two surfaces match. That match statistic expresses the amount of uncertainty in the analysis, and police investigators, jurors and others can use it when weighing the evidence. With the traditional method, an expert can only give a subjective opinion as to whether two bullets or cartridge cases were fired from the same gun. They cannot provide a match statistic.
3D systems can also make labs more efficient. In most forensic labs, the analysis done for each case is reviewed by a second examiner for quality control. This requires that the bullet or cartridge case change hands from one examiner to the next. With 3D systems, both the initial analysis and the review are done using scans of the evidence. The original items can stay in the evidence locker, and the second examiner can be in a different lab or at a remote worksite.
“The 3D approach is not yet mainstream, but many labs are looking into it,” said Xiaoyu Alan Zheng, a forensic firearms and toolmarks researcher at NIST who also worked on the new standards as an OSAC member. “There are a lot of benefits in terms of both reliability and efficiency, and the discipline is moving in this direction.”
At NIST, Zheng is developing databases of scanned bullets and cartridge cases that researchers use to study how common or rare different surface markings are on these types of evidence. These databases provide the foundation for accurate match statistics. Other NIST scientists are working on the algorithms that compare bullets and cartridge cases.
Lawton, who hopes to start implementing 3D systems at her own lab in Alabama, said that the new standards are timely.
“We are right at the cusp of this transition,” she said. “Having these standards available now, rather than after labs have already installed new systems, will help ensure that people get this right the first time around.”
In the United States, there is no regulatory authority that sets mandatory standards or requirements that crime laboratories must follow. However, many labs choose to voluntarily implement standards on the OSAC registry to improve their processes and to demonstrate their commitment to quality and accuracy.
More standards are in OSAC’s registry approval pipeline, including ones for fire and explosion investigations and toxicology. OSAC also recently started drafting new standards for sexual assault examinations.
NIST administers OSAC as part of its larger Forensic Science Program, which works to strengthen forensic science through advanced research and improved standards. NIST also supports laboratory efforts to implement standards on the OSAC registry via a cooperative agreement with the American Academy of Forensic Sciences.
Republished courtesy of NIST.