How to Find the Shooter

Approximately a third of Americans[1] are gun owners. In the age of increasingly frequent[2] mass shootings, how do forensic scientists link guns to shooters? Investigative techniques are developing rapidly, and gunshot residue (GSR) evidence is commonly used at crime scenes. However, although ammunition has often been vital in connecting criminals to crime scenes, a new type is making it harder for forensic scientists to analyse the GSR and draw useful conclusions.

Guns are usually loaded with bullets that contain elements known to be toxic, like lead. Recently, there has been an increase in the manufacture of heavy-metal-free ammunition,[3] known as non-toxic ammunition (NTA). It is much less harmful to the person using the gun (and, if used for hunting, to the environment). Unfortunately, it often has a similar elemental composition to the environment. Aluminium, potassium, iron, and silicon are some of the most abundant elements present in the earth’s crust[4] and environment.[5] It has been suggested that the most common compositions of NTA GSRs include aluminium and potassium, plus many other elements.[6] How, then, can forensic investigators use GSR to help identify bullet type and, possibly, the shooter?

If GSR is gathered quickly, it can prove incredibly useful. It lasts a maximum of six hours on the hands of a perpetrator,[7] assuming they haven’t regularly washed their hands. In reality, however, there is often very little GSR present after 2 hours,[8] even on clothes, as movement displaces the particles. This means that there is a short retention time. The composition also depends strongly on the environment and the weapon used.[9] There are plenty of studies detailing the elements present in GSR and the shape of its particles at various times after shooting, meaning there is an available database for forensics. However, most of this data is from the use of lead-based ammunition, not NTA. Unfortunately, there has not been a plethora of research on what elements make up NTA and how it is found in the environment, making identification difficult at crime scenes.

GSR analysis of toxic ammunition and NTA is commonly done using a combination of two techniques—scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM-EDX)[10]—which can identify elements in particles by measuring their energy. Each element has a characteristic X-ray energy that can be thought of as a fingerprint. SEM-EDX works because every atom has a specific number of electrons that sit at certain energy levels. An electron beam fired at the sample of GSR causes electrons to either be moved to a higher level or knocked off, creating a positively charged hole. Electrons carry a negative charge and so are attracted to the hole. As they fill it, energy is released in the form of an X-ray. This technique can therefore characterise both the shape of the particles and their elemental composition.

Fabio De Giorgio and colleagues think they may be a step closer to easy identification of NTA. In their study, they identified aluminium (Al), potassium (K), silicon (Si), sulphur (S), titanium (Ti) and zinc (Zn) in GSR particles of different NTA. These are not often found in toxic ammunition GSR, but are present individually in the environment. For example, Al is found in the earth’s crust, but Al-Ti-K-S-Cu-Zn is not a typical environmental signature. The researchers found that certain specific compositions and element combinations were found to be typical of NTA GSR, including Ti-Zn-K-Cu-Zn. Adding these to a database for forensic analysis could enable identification of the use of NTA at crime scenes.

The database cannot be immediately created, however, as sometimes toxic particles, such as lead or antimony, may still be present in the GSR. This could be the case if toxic ammunition was previously used in the same gun. Identification of the elemental composition of NTA GSR may not yet be deterministic enough to provide solid evidence of the ammunition used. However, it isn’t naïve to suggest that there could be a requirement in the future for ammunition to be manufactured with an identifiable signature. For now, though, that remains a forensic scientist’s daydream.

This research is still in the early stages and some obstacles are yet to overcome, such as the short retention times of GSR and cross contamination from toxic ammunition. However, identifying and storing the core elemental compositions of a wide range of NTA GSR in a database that is accessible to forensic investigators could prove invaluable. By comparing on-site GSR to the experimentally determined composition, many bullets and primers used in mass shootings, murders, and even suicides could be identified. This could, in conjunction with other evidence, be used to identify who the shooter was, and even to trace the guns.

 

References

  1. 7 facts about guns in the U.S., Written by John Gramlich and Katherine Schaeffer. Pew Research Centre. Accessed 22 May 2020.
  2. Sarani, Babak and Hendrix, Cheralyn and Matecki, Mary and Estroff, Jordan and Amdur, Richard L and Robinson, Bryce RH and Shapiro, Geoff and Gondek, Stephen and Mitchell, Roger and Smith, E Reed, Wounding patterns based on firearm type in civilian public mass shootings in the United States, Journal of the American College of Surgeons 228, pp. 228–234, 2019.
  3. Get the Lead Out of Bullets, By Jim Minick, The New York Times. Accessed 22 May 2020.
  4. The Most Abundant Elements In The Earth's Crust, By Benjamin Elisha Sawe, World Atlas. Accessed 21 May 2020.
  5. What Percent of the Earth is Covered by the Lithosphere?, By Susan Sherwood, Sciencing. Accessed 21 May 2020.
  6. Roman{`o}, Sabrina and De-Giorgio, Fabio and D’Onofrio, Carlo and Gravina, Luciano and Abate, Sergio and Romolo, Francesco Saverio, Characterisation of gunshot residues from non-toxic ammunition and their persistence on the shooter’s hands, International journal of legal medicine, pp. 1–12, 2020.
  7. Kara, {.I} and Yal{c{c}}inkaya, {"O}, Evaluation of persistence of gunshot residue (GSR) using graphite furnace atomic absorption spectrometry (GFAAS) method, Bulgarian Chemical Communications 49, pp. 101–108, 2017.
  8. Lindsay, Elspeth and McVicar, Michael J and Gerard, Robert V and Randall, E Dale and Pearson, Jenny, Passive exposure and persistence of gunshot residue (GSR) on bystanders to a shooting: comparison of shooter and bystander exposure to GSR, Canadian Society of Forensic Science Journal 44, pp. 89–96, 2011.
  9. Jalanti, T and Henchoz, P and Gallusser, A and Bonfanti, MS, The persistence of gunshot residue on shooters' hands, Science & justice 39, pp. 48–52, 1999.
  10. EDX Analysis with SEM: How Does it Work?, Written by Antonis Nanakoudis for ThermoFisher Scientific. Accessed 21 May 2020.

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