Transcript tool marks and impressions1

Tool Marks and Impressions
Tool Manufacturing
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When tools are made, during the manufacturing
process imperfections are left on the tools’
surface.
These imperfections are unique and specific for
each individual tool. Some imperfections are:
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Missing or partial teeth
Raised metal nodes or ridges
Distinctive signs of wear or damage
A broken tip or blade
Bolt Cutters
Forensic scientists use this information to
determine what tool left behind its mark.
The distance between a toothed instrument’s
teeth is considered a class characteristic.
Tool Marks
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Tool marks are the impressions left
on a surface when the two surfaces
come in contact with sufficient
force.
Tool marks will either be left as a
negative impression or an abrasion.
Tools
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The following tools are frequently
submitted as evidence in creating tool
marks and impressions:
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Bolt cutters
Screwdrivers and chisels
Scissors
Knives and box cutters
Pliers and wrenches
Crowbars and tire irons
Saws and knives
Places
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The following places are where tool marks
and impressions are usually found:
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Wires and chains
Door and window frames
Sections of sheet metal
Safety-deposit boxes and safes
Human bone or cartilage
Padlocks and doorknobs
Bolts and locks
Tool Mark Scenarios
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Striated or impressed marks may
be produced by tools based on the
types of actions performed. Tools
may leave a variety of combinations
of striated and impressed tool
marks as described in the table that
follows.
Tool Mark Scenarios
Action
Description
Example
Scraping
A flat-bladed tool held at a 90o to a surface and
drawn across that surface
Door or window frame with
surface scrapes from a
screwdriver or pry bar
Pinching
An opposed blade cutting tool, such as a pair of
bolt cutters or diagonal wire cutters
Remains of explosive devices
Shearing
Shear cutters, the blades of which are offset to
pass by each other in the cutting process, such
as tin snips or scissors
Cut alarm or telephone wires
Slicing
A single-bladed tool, such as a knife or axe
Tires, wires, and (rarely)
bone and cartilage
Prying
A prying tool using leverage to force open a
locked door or cover at one of its edges. May
be a flat-bladed prying tool, (e.g., crowbar,
screwdriver, tire iron, etc.)
Bank or store safe, or the
strike plate of doors
Gripping
A gripping tool with opposing jaws, such as a
pipe wrench, pliers, or a vise. Serrated jawed
gripping tools add another dimension to the
types of marks present
Doorknobs
Crimping
An opposing jawed tool designed to press
material together without cutting it
Lead seals (e.g., bank money
bags, or containers for
classified material)
Matching Tool to Tool Mark
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Since the tool has specific and
unique markings on it from the
manufacturing process, it is possible
to accurately match the tool mark
with the tool that created it.
Matching Tools with Tool Marks
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On the next following slides, an
identification procedure commonly
used by tool mark experts will be
explained.
Tool Mark Matching Procedure
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1. The expert attempts to duplicate
the original crime-scene mark by
using the suspected tool to create a
comparable mark on a similar test
medium.
Tool Mark Matching Procedure
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2. The test mark is
compared to the original
mark via microscopic
examination.
3. Patterns of impressions
or groups of striations are
matched up under a threedimensional stereoscopic
comparison microscope.
Tool Mark Matching Procedure
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4. Two dimensional
photomicrographs (photos) of the
comparison are taken for record
purposes.
5. If the marks are sufficiently
similar, the expert may conclude
that they were made by the same
tool.
Let’s Focus on Knives
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Unlike wood, metal, plastic, and other hard
surfaces, human tissue is pliable and does not
readily retain detailed marks.
Thus, knife mark analysis in human tissue
traditionally has been limited to an overall
observation of the wound itself and a microscopic
examination of the interior and exterior surfaces
of the wound to detect cuts in the skin/muscle or
the presence of fibers or other trace materials.
From this analysis, an examiner may deduce, for
instance, the general length, width, shape, or
contour of the knife blade, and the presence of
any foreign matter.
Knife Shapes/Types
Knives
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It is obvious to see from
all the different types of
knife blades that each
blade will make a unique
shape tool mark or stab
wound.
This stab wound will help
narrow down the type of
knife used, which will aid
the investigation and
narrow down the
suspects.
Stab wound from a
single edge blade
(blade on the left)
Additional Information
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Additional information can be gained from
examining tool marks and impressions.
Depending on the side of the victim’s
body that the tool marks are on will help
determine if the suspect is left or right
handed.
The directionality of the marks/stab
wounds will help determine the height of
the suspect.
Forensic investigators use all information
possible to narrow down the suspect field.