Evidence processing in criminal investigations

DEFINITION: Procedures performed in the identification, evaluation, collection, and packaging of crime scene evidence.

SIGNIFICANCE: Evidence processing is a vital procedural component of the crime scene investigation. Evidence and information obtained from crime scenes can assist investigators in identifying both victims and suspects, reconstructing sequences of events, and understanding motivations.

After a crime scene has been secured and thoroughly documented, the next step is to process the evidence. The primary steps in evidence processing are identification, evaluation, and collection of evidence. These procedures are most often the responsibility of crime scene technicians or specialists. Depending on the type of crime scene, specialists may be enlisted to collect particular types of evidence. For example, investigators may be brought in to process the evidence at scenes of suspected arson or at scenes where explosives are found.

Evidence Identification and Collection

The types of evidence identified and the techniques utilized to collect the evidence fully depend on the nature of the crime. For example, the evidence processing at a crime scene involving a is quite different from that conducted at a crime scene involving a or a drug bust. At a typical crime scene, evidence is initially identified during the search process. This very methodical process can take different forms; for example, a crime scene may be searched in a spiral, grid, parallel, or zone pattern. The objective of all crime scene search patterns is the same, however: to facilitate the identification and collection of evidence that may be helpful to investigators while minimizing the likelihood that searchers will contaminate any evidence.

After evidence is identified, it must be prioritized for collection, and the collection process must be thorough and methodical. Crime scene technicians have one chance to get it right—if evidence is overlooked, the strength of the criminal case may be significantly affected. Fragile evidence takes priority and is often collected as quickly as possible. Such evidence may include perishable evidence, fingerprint evidence, or evidence that may be easily lost. The typical types of evidence collected at crime scenes include but are not limited to trace evidence (such as gunshot residue), body fluids (such as blood), impressions (including fingerprints and tool marks), hair, fibers, weapons, clothing, tools, controlled substances, electronic data, questioned documents, and explosives.

Crime scene technicians may not always be able to identify evidence with the naked eye. After a visual inspection is conducted to identify evidence, specific lighting techniques or chemical enhancement techniques may be utilized to identify additional evidence. Trace evidence, impression evidence, biological evidence, and latent print evidence are just a few examples of the kinds of evidence that may be visible only with the aid of lighting and chemical techniques.

A number of different techniques are used in the collection of evidence. Decisions regarding which collection techniques to use often depend on the unique demands of particular crime scenes. Trace evidence, for example, may be collected manually, using lifting tape or tweezers, or with a vacuum. Trace evidence samples may also be scraped off of surfaces. I may be collected through casting, photography, or both. After latent prints have been identified, they may be collected photographically or through the use of techniques involving chemical or powder enhancement. In addition to collecting evidence, crime scene technicians must collect reference, control, or elimination samples of materials at the scene for forensic scientists to use in comparisons in the laboratory.

Packaging and Labeling

The final steps of evidence processing entail the packaging and labeling of evidence samples in preparation for their transport to the crime laboratory for analysis. It is important that crime scene technicians follow established guidelines in packaging evidence so that the samples are properly preserved. Most often, evidence samples are placed individually into various sizes and types of paper bags. Individual items must be kept separate to prevent cross-contamination. Plastic bags and other plastic containers are avoided because they facilitate the deterioration of evidence. Not all types of evidence are packaged in paper bags, however; for example, unstable liquids, such as gasoline, are typically collected in glass jars.

The general process for packaging and preserving biological evidence samples involves allowing each sample to air-dry, packaging it in a paper bag, and then refrigerating or freezing it. For example, a bloodstained shirt would be allowed to dry completely before it is packaged in a paper bag, refrigerated or frozen, and transported to the crime laboratory. When this is not possible, the evidence must be packaged at the crime scene and quickly transported to the crime laboratory, where it is then completely dried and properly packaged. In situations like this, a time limit (often a matter of hours) is usually placed on how long the evidence can be packaged while wet before it is considered useless for forensic analysis. Evidence that has not been allowed to dry completely is at risk of becoming degraded or contaminated.

After each item of evidence is collected and packaged in the proper manner, it must be labeled with important identifying information. Typically, this information includes the name of the person who collected the evidence, a brief description of the evidence, and the date, time, and location of the collection. This initial identification of the evidence begins the chain of custody. From this point on, every time the piece of evidence changes hands, the name of the individual who handles it is added to the identifying information. This detailed labeling process is intended both to prevent contamination and to prevent tampering with evidence.

Bibliography

Bennett, Wayne W., and Kären M. Hess. Criminal Investigation. 8th ed. Belmont, Calif.: Wadsworth/Thomson Learning, 2007.

English, Richard. "Who Is Processing Your Crime Scene?" Police Chief Magazine, 8 Nov. 2023, www.policechiefmagazine.org/who-is-processing-crime-scene/. Accessed 14 Aug. 2024.

Fisher, Barry A. J. Techniques of Crime Scene Investigation. 7th ed. Boca Raton, Fla.: CRC Press, 2004.

Gardner, Ross M. Practical Crime Scene Processing and Investigation. Boca Raton, Fla.: CRC Press, 2005.

Lyman, Michael D. Criminal Investigation: The Art and the Science. 5th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2008.

National Institute of Justice. Crime Scene Investigation: A Guide for Law Enforcement. Washington, D.C.: U.S. Department of Justice, 2000.

National Institute of Justice. Crime Scene Investigation: A Reference for Law Enforcement Training. Washington, D.C.: U.S. Department of Justice, 2004.