Tool marks and forensic science

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.

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 tool operational surface [ 6 ]. 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 — 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 ].

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 [ 11 ].

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 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. 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.

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, a screwdriver 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 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. 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 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.

Tool-mark analyses help forensic scientists identify the tool or tools used at the scene of the crime and link the tool s to the criminal. Investigators compare marks by examining detailed photos or silicone casts of the marks and collect trace evidence such as residue left behind by the tool.

Premise: Tool Marks at the Point of Entry Police survey the home and locate the point of entry at the back of the house. They find that the backdoor leading into the kitchen has clearly been vandalized. Pronounced tool marks are visible around the doorframe and the doorknob. They take photos and dust the door for fingerprints. While no prints are uncovered, the tool marks left on the door can be analyzed to identify the means used to enter the home.

Carefully compare, analyze and record tool mark evidence to infer a likely suspect. Per Station: 9 large cubes of modelling clay three tools, labelled A, B, and C must include one hammer Photo of tool mark evidence. Since the court case of Daubert v. Merrell Dow Pharmaceuticals, Inc. This has led to an increased drive to establish objective comparison techniques with known error rates, much like those that DNA analysis is able to provide.

This push has created research in which the 3-D surface profile of two different marks are characterized and the marks' cross-sections are run through a comparative statistical algorithm to acquire a value that is intended to indicate the likelihood of a match between the marks. The aforementioned algorithm has been developed and extensively tested through comparison of evenly striated marks made by screwdrivers.

However, this algorithm has yet to be applied to quasi-striated marks such as those made by the shear edge of slip-joint pliers. The results of this algorithm's application to the surface of copper wire will be presented.