Tool Marks and Impressions

Tool Manufacturing

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: Missing or partial teeth Raised metal nodes or ridges Distinctive signs of wear or damage A broken tip or blade

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.

Bolt Cutters

Tool Marks

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

The following tools are frequently submitted as evidence in creating tool marks and impressions: Bolt cutters Screwdrivers and chisels Scissors Knives and box cutters Pliers and wrenches Crowbars and tire irons Saws

Places

The following places are where tool marks and impressions are usually found: 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 combination locks

Tool Mark Scenarios

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 ScenariosAction Description Example

ScrapingA 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

PinchingAn opposed blade cutting tool, such as a pair of bolt cutters or diagonal wire cutters

Remains of explosive devices

ShearingShear 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 axeTires, 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

CrimpingAn 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

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

On the next following slides, an identification procedure commonly used by tool mark experts will be explained.                                                                                           

Tool Mark Matching Procedure

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

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 three-dimensional stereoscopic comparison microscope.

Tool Mark Matching Procedure

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

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

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

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.