Abstract
Background:
In criminal investigations, biological trace evidence is one of the most enduring and reliable types of evidence. The reliable identification of biological traces to the correct species in locations where humans and animals live in proximity is of vital importance to the accurate crime reconstruction and legal interpretation. Wrongly identifying nonhuman biological evidence as human biological evidence can lead to fallacies and misdirection of the investigation.

Aim:
This study analyzed the forensic biological examination of criminal cases in Al-Diwaniyah Governorate, Iraq, from 2021 to 2025. This paper focuses on crime cases, location, time of the crimes, biological traces, forensic utility, and comparative forensics of biological traces through the attribution of biological traces to human and non-human (animal) species.

Methods:
An officially documented forensic case record was used for the retrospective analysis. Crimes were categorized by type and year and then divided by geographic location. At the crime scenes, biological traces of unknown origin were recovered, and for this study, they were divided by the type of material and evaluated descriptively. In forensic analysis, if documented, species attribution was based on forensic analysis procedures, which include a morphological analysis, plus a species-specific molecular analyte (marker). The association between crime and trace types was analyzed using either Fisher’s exact test or the chi-square test, and where necessary, the effect size was calculated. The temporal data were analyzed with a count-based trend analysis.

Results:
Homicides comprised the majority of the 140 recorded criminal offenses. Presumptive bloodstains were the most frequently recovered biological traces, followed by contact traces, such as cigarette butts, clothing, and utensils. Most of the biological traces recovered at the scene were human, but records of species attribution showed that a significant number were nonhuman, mostly from domestic dogs. This finding indicates the importance of considering animal biological materials when interpreting human biological materials at crime scenes. A significant temporal decrease in the number of recorded cases was observed across the study period (p < 0.05).

Conclusion:
These cases illustrate the need for forensic scientists to determine the species of biological traces in mixed human and animal contexts. The presence of nonhuman biological materials indicates that veterinary forensics and forensic anthropology need to be incorporated into traditional forensic science although the cases analyzed were human crimes. This study highlights the importance of the human-animal interface in forensic science.

Key words: Biological trace evidence; Species attribution; Forensic genetics; Criminal investigation.