- Slides: 12
Tool Marks • • • What are tool marks? How can tool marks help make a case? If you and I own identical pry bars, how can a forensic scientist identify which one was used at a crime scene?
Tool Marks Tools make specific marks when they cut or scrape a softer surface Compression tool mark: shows outline of tool (hammer into wood) Sliding tool mark: parallel striations when tool slides across material (screwdriver or crowbar) Cutting tool mark: striations when tool cuts through material (scissors)
2 Types of Characteristics 1. Class characteristics 1. Common to a group of objects 2. Example: A hammer has a distinctive shape and size. 2. Individual characteristics 1. Unique to a given object 2. Causes? 3. Example: shoes
Types of Tool Mark Impressions 1. Static (“impressed”) marks – Caused by a harder object being pressed into another softer object – Example: crow bar into wood 2. Dynamic (“striated”) marks – Caused when a tool slides or scratches across another surface – Example: sliding a key along the side of a car (Locard? )
Types of Tool Mark Impressions, cont… 3. Cutting (“crushed”) marks – Caused by pressure being applied to both sides of an object – Examples: pliers or wire cutters 4. Multi-stroke marks – Caused by repetitive actions – Example: a saw moving back and forth
Collection of Tool Marks § Notes, sketches, photographs § If possible, the tool mark should be taken back to the lab. § If not, a cast should be made. Silicone rubber ideal material
Collecting Tool Mark Evidence • What do you think the FIRST step would be? • 2 nd step: – Bagged and taken to lab for further analysis, if possible – Make a cast of the marks, usually with silicone rubber. • What do you do with a recovered, suspect tool?
Examination of a Suspect Tool • Step 1: ? • Step 2: ? • Make test marks with the tool. • What piece of equipment would you use to compare the marks made in the lab with the marks made at the scene? • What other evidence can be obtained from a suspect tool?
Conclusions of analysis § If class characteristics and striations match, can say the tool mark was likely made by the suspect. § If class characteristics (and striations) do not match, the suspect tool can be excluded. § If class characteristics match but striations do not, the results are inconclusive.
Case Study • Leah and Milton Rosenthal: http: //www. channel 4. com/science/micr osites/S/science/society/forensic_marks. html • The Lindbergh Kidnapping