TOOL MARK EXAMINATION
Tool mark identification is a forensic science discipline that aims to identify tools as the source of marks on crime scenes. Tools leave unique marks on surfaces. This includes firearms identification which deals with marks left behind by bullets, cartridge cases and shotshell components. Examiners use comparison microscopes to compare evidence tool marks at crime scenes with test tool marks they made in the laboratory. If an examiner determines that the evidence and test tool mark are similar, a tool is then identified as the one that produced the evidence tool mark.
Tool mark identification relies on the experience, training, and education of the examiner. The most difficult aspect of this forensic science discipline is the articulation of how the identification can be determined. This process has traditionally been more qualitative than quantitative. Tool mark examiners produsce standard or test marks made in the laboratory using suspected tools to compare with the tool marks recovered from a crime scene. Test tool marks produced in the laboratory are first compared to each other to determine if the tool is producing consistent pattern. If the tool does produce consistent reoccurring pattern, the test tool marks can be compared to the evidence and see if the same mark exists. Tool marks are microscopically compared in order to identify or eliminate tools.
Types of Tool Marks
Tool marks can be classified as striated or impression tool mark. Striated tool mark refers to those marks consisting of patterns of scratches created by a parallel motion of tool against objects or surfaces. Impression tool marks, on the other hand, are produced on objects or surfaces by a perpendicular and pressurized impact of tools.
Tools have class characteristics which refer to its distinctively designed features. The rifling impressions on bullets reflecting the number, width and direction of twist of the lands and grooves in the types of barrel that fired them are an example of a class characteristic. Another example is the differences between steak and butter knife blades which results in different marks when the two types of knives are used. Some tool marks also have subclass characteristics. These characteristics arise when tools are made in batches and the manufacturing processes produced matching microscopic characteristics. In addition, tool marks also have individual characteristics. These are the irregularities on tool surfaces produced by the manufacturing processes and/or subsequent use, corrosion, or damage.
Procedures on Tool Mark Identification
Comparison and identification of tool marks are done to compare tool mark in a crime scene and test tool marks in the laboratory as well as to determine whether evidence tool marks recovered from multiple crime scenes are related to a single tool.
Prior to microscopic comparisons of tool marks, a number of procedures should be practiced and addressed. These are:
1. Reviewing laboratory protocols for tool mark examination
2. Procurement of administrative requirements (e. g. chain of custody, marking of evidence, investigative identifier, examiner identity)
3. Following laboratory safety protocols
4. Determining the evidence (e. g. paint, blood, hair, fiber, wood)
5. Determining if latent fingerprint examinations should be performed prior to tool mark examinations
6. Cleaning of trace evidence and latent fingerprints after requirements of protocols have been addressed
7. Microscopic comparison of evidence tool mark and test tool mark
a. Microscope must meet all requirements and calibration standards
b. Select appropriate magnification
c.Select the same type and orientation of light source for each comparison microscope
Misidentifications on the Sources of Tool Marks
The difficulty in tool mark examination is the identification of the one and only one tool that produced a specific tool mark. A tool may be wrongly identified because of a number of reasons. Except for firearms, databases for tool marks made by any type of tool do not exist. According to Masson’s study, misidentification of sources of tool marks may have resulted to examiners underestimating the possible similarities between the individual characteristics of tool marks created by different kinds of tools. Furthermore, examiners have not developed any rules for differentiating subclass and individual characteristics. They usually rely on their personal familiarity regarding how a tool mark is formed and how tools are made. Another cause of misidentification in tool mark sources is the change with time of a particular tool’s individual characteristics. Differences between evidence and test tool marks are sometimes attributed to the changes in the surfaces of the suspected tool between the time the evidence tool mark and test tool marks were made. Misidentification occurs if examiners wrongly attribute differences in test and evidence tool marks to changes in the same tool over time or if they fail to realize that the differences between test and evidence tool marks are produced by the same tool.