Cross-contamination of evidence

DEFINITION: Failure to preserve the purity or exclusivity of physical evidence related to a crime scene through the introduction of transferred materials from other sections of the crime scene, various related crime scenes, or other sources.

SIGNIFICANCE: Crime scenes yield evidence that can link suspects, victims, and the actions of persons present when the crimes occurred. The collection and preservation of evidential materials without cross-contamination is thus crucial, as incorrect conclusions may be drawn from contaminated evidence.

Forensic investigation of a crime scene relies on Locard’s exchange principle, which states that when two objects come in contact, they exchange trace evidence. A crime scene thus contains evidence that may place a suspect at the scene, and analysis of that evidence may reveal the associations between perpetrator and crime that are necessary for a prosecutor to obtain a conviction in a court of law. Evidence may also refute theories that link a suspect to a crime and thus may exonerate the innocent.

The methods used in the collection and preservation of evidence are intended to ensure that the preserved materials did originate from the crime scene, that the materials are pertinent to the crime, and that the materials can be analyzed in a comparable state to the way they were found at the scene. Cross-contamination of evidence results from the failure to protect an evidence sample from the transfer of other material onto or into it. Evidence may become cross-contaminated at the crime scene during collection and packaging of evidential materials, during transportation to laboratories or other facilities, during storage, or while it is undergoing analysis.

At the crime scene, cross-contamination of evidence is most likely to occur when the actions of first responders and others move materials such as hairs, fibers, and fluids around the scene. Evidence may also be compromised by cross-contamination when investigators do not use crime scene protective gear or use such gear improperly, resulting in their leaving their own fingerprints, hair, and fluids at the scene. Also, when investigators leave the crime scene to search related areas (such as a suspect’s car), evidence may be transferred from one scene to the other, resulting in cross-contamination.

Materials other than the evidential materials gathered at the crime scene may cause cross-contamination if the evidence samples are not properly packaged and safeguarded during transportation to labs or other locations. In addition, evidence must be stored properly and protected while it is being analyzed or tested to prevent cross-contamination.Technology offers improved chain of custody documentation and preservation of evidence. Examples include electronic chain of custody (eCOC) and environmental monitoring systems.

Bibliography

Alketbi, Salem Khalifa. "Maintaining the Chain of Custody: Anti-Contamination Measures for Trace DNA Evidence." International Journal of Science and Research Archive, vol. 8, no. 2, 2023, pp. 457–461, doi:/10.30574/ijsra.2023.8.2.0257. Accessed 4 Dec. 2025.

Fenoff, Roy, et al. Crime Scene Investigation. 4th ed., Routledge, 2022.

Fisher, Barry A. J., and David R. Fisher. Techniques of Crime Scene Investigation. 9th ed., CRC Press, 2022.

Genge, N. E. The Forensic Casebook: The Science of Crime Scene Investigation. Ballantine, 2002.

Pentland, Peter, and Pennie Stoyles. Forensic Science. Chelsea House, 2003.

Platt, Richard. Crime Scene: The Ultimate Guide to Forensic Science. Dorling Kindersley, 2003.