Forensic Science: Fundamentals & Investigations, Chapter 16 1

Chapter 16 Tool Marks By the end of this chapter you will be able to:

o Describe the three major types of tool mark impressions

o Summarize the steps of a tool mark examination and analysis

o Explain how technology is helping in criminal investigations

o Match tool marks with the instrument that produced them

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Forensic Science: Fundamentals & Investigations, Chapter 16 2

Introduction

o Tool mark—any impression, abrasion, or cut made when contact occurs between a tool and an object

• An example of physical evidence • Even mass-produced tools have minor

differences • The impressions can link the tool to a crime

scene and potentially to the owner

Forensic Science: Fundamentals & Investigations, Chapter 16 3

Tools and Crime Scenes

o Tools: • increase our ability to handle manual tasks, but • can also be used in crimes

o How can a tool used in a crime lead investigators to the criminal?

o Why is ownership of a tool used in a crime circumstantial evidence?

Forensic Science: Fundamentals & Investigations, Chapter 16 4

Tool Mark Impressions

Indentation Marks o Result when a tool is pressed against a softer

surface o Tools usually leave distinctive marks o The hardness of a tool influences the

resulting marks left in the softer object o May indicate the size of the tool used in a

crime

Forensic Science: Fundamentals & Investigations, Chapter 16 5

Tool Mark Impressions

Abrasion Marks o An object’s surface can be ground or worn

away by a tool o The harder object causes abrasions on the

softer surface o Indentation and abrasion marks sometimes

occur at the same time

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Tool Mark Impressions

Cutting Marks o Edged instruments can penetrate a softer

object and separate it into parts o Cut marks are produced along the edge as a

surface is cut o Different types of saws and blades leave

distinct cutting marks

Forensic Science: Fundamentals & Investigations, Chapter 16 7

Tool Mark Impressions

Examples of Cut Marks on Bones

Forensic Science: Fundamentals & Investigations, Chapter 16 8

Tool Mark Examination

o Some experts specialize in tool mark investigations

o Evidence can include: • tool marks at the scene • the tool if left behind

9

Documenting the Evidence

o Collect tool mark evidence when possible

• Cut tool mark out of door or door jamb

o Photograph the evidence with a measuring device to show scale

• Best way to preserve evidence

• Oblique lighting highlights details with shadow

• Magnesium smoking also highlights detail

o Measure the size of the impression

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Documenting the Evidence

o Castings preserve tool mark impressions

• silicone or rubber-based casting materials

o Cast impressions retain the unique indentation marks made by a specific tool

o Takes 10-30 minutes to dry and solidify.

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Documenting the Evidence

o Dust for fingerprints before applying casting material • Use magnetic dusting powder and silicone material

o The size of the impression should be measured and recorded.

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Collecting and Preserving a Sample

o Correctly label evidence o Wrap small objects with clean paper and

place them in small containers or plastic bags

o Pack large objects in cartons or boxes o Record—who, where, when, and why o Maintain the chain of custody

Forensic Science: Fundamentals & Investigations, Chapter 16 13

Analyzing Tool Mark Evidence

o Laboratory tool mark analysis identifies: • major characteristics defining the type of tool used

in a crime • unique characteristics that might distinguish

between the same kinds of tools o Forensic comparison microscopes examine

tool mark characteristics that match a suspect tool

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New Technology in Tool Mark Identification

Courts now require more than visual pattern comparisons

o Tool mark databases (with images acquired by forensic comparison microscopes)

o Algorithms to statistically analyze tool mark patterns

o Scanning tools measure the depth or height of tool marks to create a 3D image.

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Tool Mark Evidence in the Courtroom

o The tool mark witness prepares a written report to present to a jury.

o When available, provide: • Original evidence • Castings • Magnified images of tool mark comparisons

o Such evidence may link a series of crimes

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. . . . . . . . . . . . . . . . . Summary

o Tools have major and minor surface differences o Tool marks are indentations, abrasions, or cut

marks o Impressions made by tools could link them to a

crime scene and ultimately to the owner o Tool mark evidence should be photographed,

documented, and collected or cast