Tool Marks

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 MarksTools 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 characteristics1. Common to a group of objects2. Example: A hammer has a distinctive shape

and size.

2. Individual characteristics1. Unique to a given object2. 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?

• 2nd 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.channel4.com/science/microsites/S/science/society/forensic_marks.html

• The Lindbergh Kidnapping