Tool marks as evidence

DEFINITION: Impressions or abrasions made by tools when they contact surfaces.

SIGNIFICANCE: Tools are often used in the commission of crimes, and the marks made by these tools can be valuable pieces of evidence. Trained examiners can gain information about the physical specifications of individual tools from the marks they create, and these specifications can be compared with tools known to be in the possession of suspects.

Broadly defined, a tool is any object used to gain a physical advantage. Because criminal offenders often use tools to gain access to areas to which they would not otherwise have access, tool marks are commonly found at crime scenes, particularly on items such as window and door frames and safes.

Class and Individual Characteristics

When conducting tool-mark analysis, forensic scientists compare tools and their marks by examining class characteristics, which narrow down the type and perhaps even brand of a tool, and individual characteristics, which can directly match a tool to a mark. For example, if a crowbar was used to pry a window open, the found on the window frame might show that the profile of the tool consisted of two 1.5-centimeter edges with a 4-millimeter gap between the edges. These class characteristics can be used to eliminate all crowbars that have profiles that do not fit those specifications. Continuing the example, if a is identified and a crowbar is found in that person’s possession that has a profile consisting of two 1.5-centimeter edges separated by a 6-millimeter gap, that crowbar could be eliminated as the one used to pry the window open. If the suspect is found to have a crowbar consistent with the tool mark, however, further analysis would have to be performed, with the examiner comparing individual characteristics.

Individual characteristics on tools are typically the results of tiny imperfections or damage. When a tool has imperfections that were introduced in the manufacturing process, this can result in microscopic striations, or lines, in any marks the tool makes. The marks made by a damaged tool will also exhibit striations, and these are often more pronounced than striations due to manufacturing. No two tools will have exactly the same pattern of striations in their tool marks. Because of this, a microscopic comparison of striations made by a known tool to striations found in a tool mark from a can be used to determine whether the two tool marks are consistent—that is, whether they had to have come from the same tool.

Methods of Analysis

If a tool mark found at a crime scene is on an object that can be transported back to the crime lab, the object is collected. If a tool mark appears on a surface that cannot be taken back to the lab, such as a floor or wall, a cast of the mark is made so that the details of the mark can be analyzed at the lab.

Each tool mark collected is first examined for different types of class characteristics as the scientist attempts to gain information about the type of tool that was used to create the mark. If a suspect has been identified, a may then be obtained for tools in that person’s possession so that the tools can be compared with the specifications of the tool that made the evidence tool mark. Any tools identified are brought back to the crime lab and used to make tool marks in the same material as the object containing the questioned tool mark. The forensic scientist then compares the marks on a microscopic level to see if the striations are consistent with each other.

Evidentiary Value

The utility of tool-mark analysis has long been accepted among forensic scientists, but the evidentiary value of such analysis has come into question because it is difficult for scientists to assign measures of statistical significance to their findings. The difficulty in assessing statistical significance stems from a lack of defined criteria for pronouncing that a match has been made between a known tool mark and a questioned tool mark. That is, no standard has been set regarding a minimum number of striations that must be consistent for an identification to be considered definitive. Rather, all striations have to match, regardless of how many are present.

Problems have also arisen in the field as a result of a perceived subjectivity in the interpretation of tool-mark evidence. Accusations have been made for some time that the comparison of tool marks constitutes more of an art than a science. For example, in 2024, research published in the Proceedings of the National Academy of Sciences alleged that the tool mark analysis of wire cutters was too unreliable to serve as evidence. Despite these allegations, however, tool-mark analysis continues to play an important role in crime scene investigations.

Bibliography

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

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

Kumar, Sachil, et al. "Forensic Analysis and Interpretation of Tool Marks." Forensic Analysis - Scientific and Medical Techniques and Evidence Under the Microscope, 18 June 2021, doi: 10.5772/intechopen.98251. Accessed 18 Aug. 2024.

Mozayani, Ashraf, and Carla Noziglia, eds. The Forensic Laboratory Handbook: Procedures and Practice. Totowa, N.J.: Humana Press, 2006.

Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 9th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2007.

Thurman, James T. Practical Bomb Scene Investigation. Boca Raton, Fla.: CRC Press, 2006.

"Wire-Cut Forensic Examinations Currently Too Unreliable for Court, New Study Says." University of Nebraska-Lincoln, 10 June 2024, phys.org/news/2024-06-wire-forensic-unreliable-court.html#google‗vignette. Accessed 18 Aug. 2024.