Tools are often used by criminals to force entry to premises and can leave behind evidence for the forensic scientist to find.
Two tools of the same kind and made by the same manufacturer may look the same, but through use each tool can acquire differences. It is these differences that make them unique.
Forensic Scientists are able to help the courts convict criminals by matching the marks on tools to those found at crime scenes.
The examination of tool marks, as with other physical evidence, is based on two characteristics – class characteristics and individual characteristics.
Class characteristics are those characteristics that are common to a group of objects. For example, a hammer has a distinctive shape and typical size.
Individual characteristics are those characteristics which are unique to a given object. They are generally as a result of wear and tear or may be caused by isolated incidents during manufacture. For example, you buy a new pair of shoes and as you wear the shoes, over time you will get scratches and gouges on the soles. These marks are unique to your shoes.
Tool Mark Impressions
Caused by the interaction of two objects, tool mark impressions are distinguished in a variety of ways:
Static marks are made when the tool is pressed into a softer material and leaves an impression. A good example is a crow bar being used to force open a window and a subsequent impression is left in the softer surface of the wood. The Forensic Scientist will examine the marks and may be able to identify what type and size of tool caused the damage.
Dynamic marks are made when a tool slides or scratches across a surface. Think of a key being dragged along the side of a car; such an instrument leaves behind a pattern of lines or striations in the metal of the car. The pattern of striations may be enough for the examiner to identify a match with the tool belonging to a suspect.
Cutting marks are a result of pressure being applied at both sides of an object and are often associated with scissors, wire cutters or shears. When used these tools can leave behind marks and striations along the cut edges of the material.
Multi-stroke marks are caused by repetitive action, such as a saw moving back and forth.
The Scene Examiner will examine and photograph the tool marks in situ. If appropriate, the Scene Examiner will remove the object with the marks and take it to the lab for further analysis. If this is not possible they will make a cast of the marks, generally using a silicone rubber.
A tool may be recovered that is suspected to have caused the damage and this will also be taken to the lab for further analysis.
The Forensic Scientist will make test marks using the suspect implement. The test mark and the mark recovered from the crime scene will then be compared.
The Forensic Scientist will examine and compare the striation patterns using the comparison microscope. By comparing and matching the striations the scientist can prove whether the tool is responsible for the impression.
Tools can also have trace evidence, such as paint flakes adhering, or in the case of a human victim, blood or other body fluids. This evidence greatly assists in the investigation of a crime.