Case Grabber CSI tool
by Alexander Jason, MFS, CSCSA, F-AAFS
DNA analysis of fired cartridge casings has become an increasingly valuable tool in forensic investigations. While in the past there have been challenges in the DNA processing, recent advances in collection, packaging and transport methods have demonstrated high potential for recovering usable DNA profiles from these pieces of evidence.
The technology for effective DNA processing of cartridge casings has made great advances. A comprehensive study1 published in 2022 showed that with a new recovery method, approximately 67% of fired casings recovered at crime scenes produced a usable DNA profile. Another older study2 done in 2015 by the San Diego Police Department, which used a less effective processing method, showed a success rate of 30%.
Traditionally, casings have been examined for fingerprints (rarely successful) and tool marks, which can link the casing to a particular firearm. However, DNA analysis offers another powerful avenue for identification that should not be overlooked. DNA has the potential to directly link a suspect to the crime scene. When a firearm is loaded, the person handling the ammunition may leave trace amounts of DNA on the cartridges through skin cells or other biological material. If this DNA can be recovered and profiled, it could place a suspect at the scene of the shooting and/or connect them to the weapon used.
In cases where there are no known suspects, DNA profiles obtained from cartridge casings could be run through DNA databases to potentially identify the shooter. Even partial profiles may provide investigative leads or connect multiple crime scenes involving the same unknown perpetrator. In a studyof Shotspotter activations, no suspects were located at the identified locations, but fired casings were found. In the San Diego study, the collection and DNA processing of those recovered casings led to the identification of a shooter.
Advancements in DNA recovery techniques
Early attempts at recovering DNA from fired cartridge casings yielded limited success. However, recent studies and technological advancements have dramatically improved the potential for obtaining usable DNA profiles from this challenging evidence type by utilizing enhanced extraction and amplification methods. Advances in DNA extraction kits and amplification methods have increased sensitivity, allowing for the detection and analysis of smaller quantities of DNA. Some labs have implemented modified protocols specifically optimized for low-template DNA samples like those found on cartridge casings.
Additionally, the development of probabilistic genotyping software has greatly improved the ability to interpret complex, low-level, and mixed DNA profiles, enabling more meaningful analysis of the challenging samples often recovered from cartridge casings.
New collection devices and extraction protocols have been specifically designed to maximize DNA recovery from the challenging metal surfaces of cartridge casings. Some key improvements include:
- Using a combination of different swab types and rinse solutions to more effectively collect trace DNA from the casing surface
- Incorporating additives in the rinse solution that reduce the degradative effects of copper on DNA
- Employing techniques like soaking and sonication to dislodge more cellular material from the metal.
In the San Diego P.D. study, 96% of samples had the loader’s DNA, while 40% of samples were mixtures. By utilizing soaking techniques for DNA recovery, forensic lab technicians were able to produce interpretable DNA results for 35% of the overall samples.
Collection and packaging
One cause of poor DNA recovery from fired casings tests is improper handling, packaging, and transport of the casings. It has been common to place casings in paper envelopes or small cardboard boxes for transport from the crime scene to the lab, but these types of packaging create conditions in which the casing can rub against or otherwise contact the interior surfaces. One test showed that 56% of the DNA was lost3.
“Overall, this study demonstrates that DNA transfer (i.e., loss) within packaging is not only a possibility but, under certain conditions, a very likely occurrence,” the study authors concluded.
Given these results, some manufacturers have redesigned casing packaging and transport to avoid the problem. For example, the patented Case Grabber tool for crime scene investigators ensures safe collection, packaging, and transport of fired casings in a unique way. The casing is retained and collected from the inside by a specially designed stainless steel spring tweezer, after which a clear plastic container (tube) is snapped on the device handle, protecting the casing surface from contact. The casing is securely retained, and the tool with the protected casings is transported safely to the lab. Once ready for processing, the protective tube can be removed and the casing can then be swabbed, etc.
While the potential benefits are clear, there are several challenges and considerations to keep in mind when swabbing fired cartridge casings for DNA.
Best practices for DNA recovery from cartridge casings
To maximize the chances of recovering usable DNA from fired cartridge casings, several best practices should be followed:
Cartridge casings should be collected as soon as possible after the shooting incident to minimize environmental degradation of any DNA present.
- Proper Handling and Packaging
Crime scene technicians and evidence handlers should use appropriate personal protective equipment and avoid touching the surfaces of the casings to prevent contamination. Picking up the casing from the inside is an effective method. Evidence packaging should be restricted to methods and materials that will prevent contact with the casings’ outer surfaces during transport.
A double swab technique, using one wet followed by one dry sterile swab, has been shown to be effective for sampling the entire surface of the casing.
Labs should use DNA extraction methods optimized for low template samples, potentially including additional steps like spin basket use to maximize DNA recovery. Highly sensitive PCR amplification kits designed for touch DNA samples could be employed to increase the chances of obtaining usable profiles.
The use of probabilistic genotyping software for interpretation can also help maximize the evidentiary value of complex, low-level DNA profiles often obtained from cartridge casings.
Future directions
As technology continues to advance, the potential for recovering DNA from fired cartridge casings is likely to improve. Some promising areas of research include techniques that allow for DNA amplification directly from the evidence item, bypassing the extraction step—which could increase sensitivity and reduce the risk of sample loss.
As success rates improve and technology advances, cartridge casing evidence is likely to become an increasingly valuable tool in criminal investigations. By following best practices—including the use of collection, packaging, and transport tools that protect exterior trace evidence—and staying abreast of new developments, forensic laboratories can maximize the chances of recovering crucial DNA from these small but significant pieces of evidence.
The ability to link a suspect directly to a crime scene through DNA on a fired cartridge casing could be the key to solving otherwise challenging cases. While not every casing will yield a usable profile, the potential to uncover critical evidence in even a fraction of cases justifies the effort of routine DNA swabbing of fired cartridge casings.
About the author: Alexander Jason holds a Master of Forensic Sciences degree (Honors) from National University and is a long-time Board-certified Senior Crime Scene Analyst. He is also a Fellow of the American Academy of Forensic Sciences and a Technical Advisor to the Assoc of Firearm and Toolmark Examiners. Jason's research has been published in many forensic journals including the American Journal of Forensic Medicine and Pathology and the FBI's Forensic Science Communications.
References
1. Recovery of DNA from fired and unfired cartridge casings: comparison of two DNA collection methods, Elwick, Kyleen et al., Forensic Science International: Genetics, Volume 59, 102726
2. Montpetit S, O'Donnell P. An optimized procedure for obtaining DNA from fired and unfired ammunition. Forensic Sci Int Genet. 2015 Jul;17:70-74. doi: 10.1016/j.fsigen.2015.03.012. Epub 2015 Mar 25. PMID: 25828369.
3. Mariya Goray, Roland A.H. van Oorschot, John R. Mitchell, DNA transfer within forensic exhibit packaging: Potential for DNA loss and relocation, Forensic Science International: Genetics, Volume 6, Issue 2, 2012, Pages 158-166, https://doi.org/10.1016/j.fsigen.2011.03.013.