A criminal often makes use of varieties of tools in the commission of the crime. Among them, knives and screwdrivers are the most common tools because of their general utility, availability, and ease of concealment. Bits, jemmies, saws, pliers, cutters, hammers, scissors, drills, and tools specially designed for burglary like Sendh Katti are also frequently encountered in criminal investigation work.
A tool is made of hard metal to enable it to operate effectively on softer material. It thus leaves marks on the surface, on which it operates, without damaging the tool. If the tool is not extensively used or misused and is properly maintained, the marks are reproducible almost indefinitely. Thus, a criminal can be linked to a crime even after a long interval through the tool and tool marks.
A culprit usually uses the same set of tools in the commission of similar types of crimes. The crimes can be linked if tool marks from various crimes are compared and found identical. Tool marks are, therefore, of great importance in the criminal investigation.
Tool Mark Examination
Tool mark examination is a term that includes a wide variety of impressions that are not necessarily directly related to tools but that are created in the same fashion and are, therefore, examined with the same techniques. A clear example is an impression left by a firearm’s barrel onto a bullet or by the firearm onto the cartridge. These are a specialized category of tool marks. The impressions produced by human teeth, as well as the impressions left by shoes or tires, are other examples. The tool mark examiner is frequently the person in charge of inspecting and offering professional views on such impression identifications.
Collection of Evidence bearing Tool Marks
The original articles bearing the questioned tool marks should be collected. If it is not possible, then casts of tool marks in sulfur, plasticine, dental cement, etc, should be obtained. While collecting the tool it is important to preserve the traces on it such as paint, oil, metallic residues, etc. These traces are as valuable as the tool and tool marks.
If pieces of wire that have been cut are to be sent for examination, the suspect end of the wire should be marked. The suspect tool must not be used to cut these wire ends.
Examination and matching of Tool Mark
Examination and comparison of the tool marks from a crime scene with the tool marks of the actual suspect tool can act as important ad invaluable evidence in linking the suspect to a particular crime and thus the case can be concluded.
The major precautions that are to be taken while examining the tool marks:
- Door and windows and other openings with handles or locks at the crime scene should not be touched if they are broken or the locks are cut, lest the tool marks or the fingerprints are destroyed.
- A tool should not be fitted forcibly into the impression which may affect the laboratory analysis.
- The tool marks should be documented completely including sketches and photographs before removing them.
- If any trace evidence is found on the tool marks, the examination of the trace evidence should be done before the tool marks examination.
- Tool marks evidence should be packaged so that it is not damaged as it may change the microscopic characteristics.
- If the tools have been stained by blood or other biological material, they must be cleaned with a soft bristle brush and disinfectants such as Terg-A-Zyme, ethanol, and other similar products.
- The tools are always cleaned with cotton-tipped swabs saturated with ethanol or acetone.
Principle of Comparison of Tool Marks
Practically, it is impossible to manufacture two tools that are exactly alike. Most of the tools used in crime are not manufactured under controlled conditions. As a result, they frequently exhibit both gross and microscopic distinctions. Their frequent use causes wear, dents, and other damage. Reshaping tools will add new characteristics or change existing ones, allowing the tools to be distinguished from one another.
Preparation of Test Marks
It is very difficult to reproduce test marks of the tool even if they are made of similar materials, in a similar manner, and with a corresponding position and portion of the tool. It is, therefore, essential to pay utmost care and attention while making proper standards. The test marks are made on a soft material so that the tool edge does not get altered. When the original mark is in wood, paint, or soft metal, the standard should be made with the same material. When the original mark is on hard metal, such as steel, it is not desirable to use the same material as standard. In such cases lead or tin sheet may be used for test marks.
Comparison of Marks
The standard and questioned marks are compared usually under the comparison microscope. Due to wide variation in tool marks, the presence of a clean-cut match of a reasonable portion of the questioned and the standard marks can be virtual proof that the same tool made both marks. The marks, under identical conditions of lighting and positioning, look like a series of parallel lines of variable width and arrangement. If both marks resemble a considerable degree, then they are said to be identical.
Physical fits, sometimes known as “mechanical fits,” can be found in a wide range of criminal investigations, sometimes even as part of a more detailed tool mark. When it’s critical to confirm that two or more parts of a product that have been partitioned, broken, cut, and often violently removed are linked or fitted together, a physical fit exam is required. The objective scientific reliability and admissibility of such physical comparisons are viewed with skepticism throughout the trial, according to the restricted physical comparison literature studied.
Physical fit examinations are divided into these main categories:
- Broken parts that may be repaired, often known as “jigsaw” suits.
- Broken items require a complete tool mark investigation, including microscopic comparison and casting, to verify that the parts fit together and so draw an inference.
- Broken, torn, or split things that require knowledge of the manufacture and the appearance of marks left on the material’s surface to facilitate a fit.
Some of the most important aspects of forensics are the collection, processing, and evaluation of imprint and tool mark evidence. The disadvantages of the selection and preservation criteria are unacceptably mixed, generating a bad perception of resources and environmental factors. These limitations occur due to lack of information, a compromise in class perception, and the individualization of attributes utilized to locate a specific piece of evidence at a crime scene. With the inherent destructive potential of impression and tool mark casting, it is vital to make the best cast possible first.
Occasionally, the circumstances of a crime scene impact the availability of casting techniques, contributing to the continuity of a silicone rubber casting material is supplied as a partly polymerized base with which a catalyst must be mixed to allow polymerization. Microcell Silicone Casting Medium is used by forensic investigators to duplicate the tiniest tool traces and impressions. They are superior to other recognized flexible silicone casting procedures in terms of visibility of tool marks, firing pin impressions, and latent fingerprint lifts, according to laboratory investigations.
- An image of the tool’s impression is taken, which serves as a permanent record of some of the tool’s properties.
- If the carrier or the object containing the tool mark cannot be gathered as evidence, a dental paste cast of the tool mark is prepared.
- The comparison process begins after the tool is discovered and its class features match those displayed by the tool mark under evaluation.
- The tool is examined, a photograph is taken, and then softer surface comparison tool markings are created.
- A comparison microscope, which consists of two microscopes connected with which two objects can be observed simultaneously with the same degree of magnification, or a stereomicroscope is then used to compare the comparison tool marks and the tool markings under study.
- If there is a match, the common origin of the two tool marks is determined, which leads to a clear case report.
A tool mark is an impressive mark that occurs when a tool and a surface come into contact with enough pressure. The striated surface of the “tool” is compared to the striated surface of the questioned tool mark during the examination process. Additional evidence, such as trace hair, fiber, and fingerprint evidence, can be examined by our scientists.
The goal of the tool mark analysis and comparison is to see if a mark or a sequence of markings in dispute were created by a certain tool. A thorough study of the disputed tool mark(s) usually yields details of the tool’s class and size, as well as if the tool is damaged and how it was used to make the alleged marks.
The morphological evaluation of the tool and its features is the first step in a tool mark analysis. As a result, a categorization method for each tool mark, such as branding, cutting, compression, crimping, engraving, firing, and so on, is suggested to supplement the typical classification systems found in the literature and to give a more standard way. Tool mark analysts are objective and conceptual analysts that try to figure out if they’re dealing with a tool mark or something comparable.
An automated tool mark identification system uses an acquisition method for the processing of three-dimensional data from tool marks left by tools on the sample surface. A signature generation module for the generation of tool mark signatures from the data collected, and an analysis unit for the comparison of pairs of tool mark signatures to obtain a numerical similarity value representing their identical characteristics. The procedure is carried out with the aid of an integrated computer.
The tool marks which are the impressions left by a tool at the crime scene play a vital role in discovering and identifying the tool used to commit the crime by comparison processes. These tool marks have to be documented carefully and completely including the physical features of the tool and tool marks, lest some information is destroyed. The tools under examination should also be cleaned with a soft cloth dipped in ethanol. The examination process is carried by comparison microscope and stereomicroscope.
A comparison microscope is a tool for examining multiple specimens at the same time. It comprises of two microscopes joined by an optical bridge, resulting in a split view window that allows the examination of two separate objects at the same time. When connecting two things under a conventional microscopе, the viewer no longer has to depend on memory.
Fiber optic illumination, video capabilities, digital imaging, automatic exposure for conventional photography, and other optical, mechanical, and electronic enhancements are included in the modern apparatus.
The stereo or dissecting microscope is a type of optical microscope that uses incident light illumination rather than transillumination to observe a material at low magnification. It used two different optical pathways, two eyepieces, and two objectives to provide the left and right eyes with slightly different viewing angles. A three-dimensional imagining of the sample is produced in this way.
The stereo microscope is commonly used to examine the surfaces of solid specimens or to perform close tasks like sorting, dissection, microsurgery, watch-making, small circuit board manufacture, and inspection of other things.
The stereo microscopе should not be mixed up with a compound microscopе. It is fitted with double eyepieces and a bino viewer. In such microscopе, both eyes get the same image. But in the binocular eyepieces deliver greater observing comfort. The image obtained with such a microscope, however, is identical to that obtained with a single monocular eyepiece.