Police vehicle contamination by inorganic gunshot residue (iGSR) in Zagreb County Police Administration (Croatia): Analysis of characteristic and indicative particles across different vehicle parts and contributory risk factors

Inorganic gunshot residue (iGSR) analysis, crucial for linking suspects to firearm use, faces challenges from potential environmental contamination, notably in police vehicles. The present study aimed to explore the level of iGSR contamination in police vehicles from the Zagreb County Police Administration (Croatia), considering particle types and their position in vehicles, and to identify associated risk factors. From December 2021 to April 2022, 65 of 86 police vehicles (margin of error: ±6% at a 95% confidence level) were sampled with GSR stubs on the drivers' seats, back seats, and backrests and analyzed using scanning electron microscopy and energy dispersive x-ray analysis (SEM/EDX). Characteristic particles were found in 63.1% of vehicles, 33.8% on the driver's seat, and 24.6% on the back seat/backrest. Indicative particles were found in 70.77% of vehicles, with a fairly even distribution. McNemar's chi-square analysis showed no significant disparities in positive sample ratios across vehicle parts or particle types (p > 0.05). In total, 228 characteristic and 166 indicative GSR particles were identified, with no notable correlation among them (p = 0.346). Logistic regression analysis identified the transportation of individuals involved in firearms incidents as a statistically significant factor influencing the presence of characteristic particles (p = 0.030). The findings suggest a considerable prevalence of iGSR in the analyzed Police Administration unit, highlighting the need for careful contamination management in police operations to preserve evidence integrity, particularly in cases when individuals who used firearms had been transported in the vehicle.