The forensic analysis and interpretation of tool marks raise for consideration key methods and advances in the field of tool marks in forensic science. This chapter shows how tool mark analysis can be utilized in the course of criminal investigations. The focus of the chapter is on bringing together as much scientific knowledge in the area as possible in an accessible manner. It covers all aspects of tool mark evidence from the crime scene to the courtroom. This chapter provides information about tool marks in an effort to assist tool mark examiners as well as people practicing forensic science, crime scene examiners, crime investigating officers and members of the legal profession. It includes information about the analysis of tool marks at the crime scene and in the laboratory, the interpretation and assessment of challenges for examination and interpretation and also the way in which tool mark evidence can be presented in a courtroom.


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1. Introduction

Tool mark identification is a fascinating forensic science discipline. By comparing the pattern of the tool marks in question and the pattern of the tool marks generated by the tool in a laboratory environment, a skilled analyst can give an opinion based on the accuracy of the questioned tool mark produced by a specific item <1>. This assists the forensic investigator in matching the marks on tools to crime scenes. Forensic tool mark identification includes firearms identification, an area of tool mark investigation that specializes in identifying different firearms and parts of a firearm being used at crime scenes. It also includes fracture matching or a physical fit <2>, whereby two specific objects are analyzed to determine whether they have been at one time a single unit. If that is the case, the investigator will further analyze how the two objects come into contact and how they affect each other.

Tool marks can be generally understood as impressions or marks that are produced by a tool <3>. When a tool contacts a surface with sufficient force, a mark or an indentation is permanently left on the receptive surface.

A striation, as defined by AFTE, is a range of marks on the surface of an object <4>. These marks are produced by a combination of impact and motion. A pry mark made by the tip of screwdriver is a type of striated tool mark <2>. Similarly, an impression can be defined as a range of marks on the surface of an object <4>. As with a striation, an impression is produced by a mixture of impact and motion. Notably, impressions are not caused by strong impact but appear on a surface as soft or shallow indentations. A hammer impact is a type of impressed tool mark.

Tools may be connected to tool marks and vice versa due to certain patterns or anomalies during the manufacturing process embedded in their surface. It is argued that patterns and anomalies of the tool mark are specific to each tool; the distinguishing features of a particular tool may be one aspect, just as the markings on a bullet can lead to a particular one and can be identified and compared visually. In consideration of this, a forensic investigator can become familiar with the manufacturing processes used to manufacture the working surface <5> of a tool and can compare the class features with the same surface of the tool such that it is possible to measure the uniqueness of a tool and its tool mark. Knowledge and understanding of tool manufacturing methods, along with close examination of tools and markings of tools, will make it easier to carry out this particular recognition.

There was no direct way in the past to associate a tool mark with the tool itself, and little progress has been achieved with the advent of modern forensic technology. In using tools to gain entry, a burglar will invariably leave tool marks behind that are of forensic significance and potentially incriminating, which can provide vital evidence to investigators and prosecutors. Given this, the essential factors that influence both tool mark production and the subsequent inspection of such marks in the forensic examination can be determined. These factors include the following:The surface material that the tool is functioning on

The material used in order to construct the tool

The relative hardness of each material

The manufacturing procedure followed in order to construct the tool

2. History of tool marks

Since many previous centuries, a historical understanding of the tool mark has been recognized that marks can be connected directly to tools, but few written references are typically found on this specific subject. A cited example often comes from China in the Twelfth century, where various wound shapes created by cutting tools such as sickles were considered, but even in China, there is little evidence of their importance.

Henry Goddard (1800–1883) of Scotland Yard is remembered as the first investigating officer to collect forensic evidence by analyzing a bullet and its related pattern to investigate a murder <7>. In 1835, using a bullet recovered from the autopsy victim’s body, a defect was discovered that could be traced back to the original mold from which the bullet was made. In 1891, Hans Gross published a book entitled “Handbuch für Untersuchungsrichter als System der Kriminalistik” detailing all the basic precautions for the analysis of tool marks <8>.

In 1953, a popular book entitled “Crime Investigation” textbook written by the renowned criminalist, Paul Leland Kirk (May 9th, 1902 – June 5th, 1970), explains the need for cast marks found in crime scenes if the item with the mark cannot be transported to the laboratories and makes a strong distinction between “compression marks” and “sliding marks.” In his book, he examined immersed marks by using macrography while comparison microscope was used to analyze striated marks, along with the examination of physical fit. In the 1974 edition, there is a reference to the work by Biasotti <9>, The Principles of Evidence Evaluation” as applied to Firearms and Tool Mark Identification, which contains some of the first references for objective methods for evaluating striated marks.

In 1958, a book entitled ‘An Introduction to Tool Labels, Weapons and the Striagraph’ was written by John E. Davis, a prominent criminalist and the chief of the Oakland Police Department (CA) Criminalistics Division (Crime Lab). This textbook also introduced a new advanced piece of research equipment called “Striagraph,” which was able to calculate, trace and record microsurface contours and was the precursor to advanced laser and digital imaging techniques for future bullet surface scanning technology <10>.

The Association of Firearms and Tool Mark Examiners (AFTE), an international nonprofit organization devoted to facilitating the identification of firearms and tool marks, was founded in the United States in 1969 <11>.

3. Definition of tool marks

Tools are mostly directly related to object markings, because at the time of tool production, such designs or irregularities are imprinted on their surface, so it is implied that these patterns and variations might be part of the identification features of a particular object; for example, marking bullets can lead to a particular firearm. Furthermore, these substantially different types and irregularities of the instrument can be visually identified and compared using forensic techniques <12>.

The term “tool mark” is defined in a number of ways. A widely accepted AFTE definition defines tool mark as “If any object or instrument reaches the surface with enough force to allow its signature design to be indented, this form of marking is referred to as a tool mark.”. In another definition <6>, it is stated that “An instrument that is considered to be sufficiently stronger from two objects acquires comprehensive force when it comes into contact with each other, which leads to the softer one being marked.”.

Biasotti and Murdock <13> state that “When two objects begin to interact, the extremely hard object will stamp the surface of the softer object. The relative hardness of the two artifacts, the pressures and motions, and the appearance of the microscopic discrepancies on the object are all factors influencing the character of the generated toolmarks.” It is necessary to establish the correlation between a tool mark and the tool that produced it in criminal investigations such as burglaries. For instance, if a burglar chooses wooden or metal bars to force entrance into a home, the marks left by the tool on the doorway are strong evidence of the involvement of that tool for that legitimate purpose at the scene of the crime. If the tool is linked with, or close to, a suspect, it enables for the identification of a link between the accused person and the incidence of the crime.

4. Types of tool marks

Generally, there are three categories of tool marks left by tools on the surfaces they hit. These impressions are produced by the possibility of a compression action, sliding action or cutting action occurring.

A compression impression: Probably the most common and most negative representation of the surface of the tool, caused by pressure, blow or gouge of the tool on the surface of a wood, metal or other surface. Compression is imprinted on softer material when tool surface presses against its surface <14>. For instance, ascrewdriver is most often used to tighten or loosen screws. However, if it is used to pry open a widow, it will leave impressions in the windowsill.

Friction marks (sliding action): The second type is a mark of abrasive wear or resistance left by the tool’s sliding or chopping action that creates striations on a marked surface. Friction marks are fine parallel striations and are a characteristic feature left by a tool scraped across a smooth surface, such as dressed wood or metal. It is common to focus on such striations when making bullet and tool mark comparisons. Parallel lines have the potential to be matched using microscopic comparison. There are an infinite number of ways to apply a tool to a surface, and the resultant striations are the effects of every variation. For example, when a crowbar is forced into the area between a door and the front part of the door to force the door wide open, pressure is applied to the tool handle. An abrasion or friction mark is created by forced application of the crowbar. The majority of bull cutter marks on rods or wires, screwdriver scratch marks and knife or axe cut marks are examples of friction mark markings.

Cutting edges are not as commonly used in the commission of crimes as prying tools with blunt edges, so finding marks of cutting tools is not frequent. There is a high significance in cut marks being positively identified with the tool producing them. A cutting impression is a combination of these two impression types, as is found in scissors.

From these three tool mark impression types, both the class and individual characteristics of the tool can be identified; for instance, marks left on a doorway from a pry bar can be matched back to that specific pry bar.

During tool mark analysis, the analyst may discern what type of tool made a particular mark, and whether a tool in evidence is the tool that made it. The tool mark can also be compared to another tool mark to ascertain if the marks were made by similar, or the same, tools.

5. Types of tool marks comparison

A well-known and extensively used forensic methodology is the comparison of tool marks, which is typically regarded to provide convincing trial evidence and facilitate the investigation of a crime. However, there is a great deal of ambiguity as to the uniqueness of such marks and, in particular, the probability of more than one tool replicating a mark. According to Houck and Siegel <15, 16>, tool mark examiners need to have a conceptual understanding of how to produce and machine a variety of tools. Limitations on comparative forensics have initiated the need for an objective, as each tool has specific surface characteristics for the identification of tool marks to facilitate scientific research. In 2009 National Academies report, researchers recommend reinforcing the scientific justification for the standards and specifications for the tool mark identification in forensic science.

The forensic principle of comparison explains that only the like can be compared with the notion of comparison. It reinforces the need for samples and specimens to be included for comparison with the objects in question. Therefore, the prime purpose of forensic comparison is to establish which characteristics and specifications of the samples in question obtained from the crime scene (including a tool or a population of reference items, screw bag or plastic bag roll) varied or directly correlate with those obtained from the source on the control item. Comparing features, however, is a deceptively simple process, but understanding what the outcome implies is much more difficult if one does not understand exactly what the characteristics and specifications are or how they were acquired.

Another challenging part of a comparison is to examine the manufacturing patterns associated with the “control” object. The manufacturing process leaves distinctive microscopic striations on the tool’s operating surfaces as the marks produced depend not only on the type of tool being used but also on how it is used (as a hammer, or lever or force exerted), the contact position (leading angle or trailing angle) and other factors that may help to identify the metal tools <16>.

There are three categories of features that an examiner will need to identify:Individual/unique characteristic: Individual characteristics relate to the specific characteristics of both the questioned samples and the reference samples, which share a similar origin with a high degree of reliability. Examples of evidence possessing individual characteristics are fingerprints, tool marks and markings on bullets.

Therefore, in order to analyze the results, it is imperative to understand the sets of features and details generated during the production process and then use, how they will be portrayed in a mark and how to differentiate between the different types, as this will determine what you can say about the comparison. The quality of the situation mark in a mark comparison is always the main limitation. Information that may have been visible on a tool may not have been replicated in a mark for certain variables, such as the physical parameters of the material. If they are considerably weaker than the tool, the information of interest cannot be replicated completely.

However, occasionally, the difference will be significant and on occasions may even be to the extent that one expert will say the tool was responsible and the other that it was not the tool. Occasionally, while the difference is apparent, it may be to the degree that one analyst states that the tool was accountable and the other that it was not the tool. With all this perspective, the importance of the independent critical results test of a secondary tool mark expert should not be overlooked. However, this is not always necessary, and in order to settle the debate, a third expert may be required to conduct a verification.

AFTE Theory of Identification (1998) classified four categories of tool examination:Identification is the inference that the class traits of two samples appear to be the same and that the individual features are reasonably agreed to conclude that the same weapon was shot. If they agree, for instance, two copper jacketed bullets are found.

Inconclusive agreement of class characteristics is defined as “the outcome of a comparison in which there is some agreement of individual characteristics and all discernible class characteristics, but insufficient for identification, agreement of all discernible class characteristics due to an absence, insufficient, or lack of reproducibility, agreement of all discernible class characteristics and disagreement of individual characteristics but insufficient for an elimination”.

A substantial disparity between distinguishable class characteristics and/or individual characteristics is triggered by elimination, or exclusion from the analysis. For fired bullet comparisons, an exclusion is usually based on observed differences in some of the general rifling properties.

In the absence of microscopic marks, “Inappropriate for comparative analysis,” appears.