Unit 11: Toolmarks & Impressions Notes

Tool Marks and Impressions

Introduction to Tool Marks
  • Definition: Tool marks are impressions left behind by tools during the commission of a crime. They are often found at points of entry into structures or on the evidence itself (e.g. safes).
  • Importance: Tool marks can provide evidence linking a suspect to a crime scene based on the unique characteristics of the tool used.
Characteristics of Tool Marks

  • Class Characteristics: General features of the tool that can be categorized.

    • Examples: Type (screwdriver), Size (1/4”), Shape (flat-head)

  • Individual Characteristics: Specific features that can indicate wear and damage to the tool.

    • Examples: Pitted edges, scratches on the tool that leave distinctive marks on the surface.
Comparison Microscopy
  • Utilized for comparing tool marks from the crime scene with test impressions created from suspect tools.
  • A comparison microscope allows forensic analysts to visualize differences and similarities between marks from a known suspect tool and the evidence.
Preservation of Evidence
  • Guidelines for Handling Tool Marks:
    • Whenever possible, submit the entire object or part carrying the tool mark to the crime lab to maintain evidence integrity.
    • Avoid fitting a suspect tool into the tool mark onsite, as this can alter the evidence and compromise its validity.
Other Impressions - Shoe, Tire, Fabric

  • Different types of impressions, such as shoe, tire, and fabric, can also provide crucial evidence.

  • Photographing Impressions:

    • Essential to photograph impressions including a scale before handling.
    • If impressions appear on easily recoverable evidence (glass, paper), transport them to the lab intact.

  • Fabric Impressions:

    • Example Case: A fabric imprint on a suspect's vehicle led to a confession in a hit-and-run case.
    • Comparison of clothing weave patterns can be decisive in linking suspects to crimes.
Techniques for Evidence Recovery

  • Photographic and Casting Techniques:

    • Capture footwear impressions in soft earth through photography and casting.
    • For faint blood impressions left by shoes, chemical treatments can enhance visibility.

  • Casting Process:

    • Use aerosol hairspray to harden impressions, mix dental stone to a batter-like consistency, and pour into the impression to create a cast.
Chemical Treatment of Bloody Impressions
  • Various chemical agents can enhance bloody shoe impressions for forensic analysis:
    • Amido Black, Hungarian Red dye, Leucocrystal Violet, etc.
Points of Comparison in Impressions
  • A sufficient number of distinct points can validate that questioned and test impressions originated from the same source.
  • SICAR Software: A forensic program that assists in making shoe print comparisons.
Class and Individual Characteristics of Impressions

  • Shoeprints:

    • Class characteristics include manufacturer, type, model, and size.
    • Individual characteristics encompass unique wear patterns or embedded objects.

  • Tire Impressions:

    • Similar to shoeprints; class features reflect design, size, and type while individual features relate to wear and damage.
Bite Mark Impressions
  • Often found in cases of assault; can serve as individual evidence when sufficient impression remains.
    • Notable Case: Ted Bundy murders provided forensic odontologists the ability to match bite marks to Bundy's dental impressions.
Conclusion
  • Understanding tool marks, impressions, and their characteristics is essential for forensic investigations and linking suspects to crimes. Accurate preservation and analysis methods are crucial in collecting and evaluating this evidence.