Footwear and Tire Impression Crime Scene Recovery

Footwear and Tire Impression Crime Scene Recovery

Contributed by Lesley Hammer

The purpose of footwear and tire track impression crime scene recovery is to document the location and orientation of these impressions and to provide an accurate representation of the features of the impression for examination by a forensic physical impression expert.

Certain criteria must be met for fracture match examinations to be conducted. Obviously the item(s) must be broken and detached, then the items must be capable of being physically realigned (perhaps a portion is missing, can it be found), do the items fit together as a “lock and key” like a jigsaw puzzle (surface markings may exist on the item that align and look for an edge to edge boarder), and are the pieces unique (can they be interchanged with similar pieces elsewhere).

When a person or a vehicle travels to and/or within a crime scene, impressions of footwear and/or tires are left. Footwear and tire impressions are typically classified as 2- or 3-dimensional. 2-dimensional impressions are made when a substance is deposited on or removed from a surface by a footwear outsole or tire tread. 3-dimensional impressions are made when the sole or tread is impressed into a substance such as snow or mud. Documentation and collection of footwear or tire impression evidence is accomplished through the use of photography and other processing techniques such as lifting, casting and chemical development.

Photography

A high quality photograph and specialized lighting techniques are utilized to capture sufficient detail. A variety of photographs are taken to document the impressions within the scene as well as close ups for enlargement and examination. Close up photographs for examination should include a scale, with the camera positioned directly above and parallel to the impression. Photographs are taken of impressions before other techniques for collection are applied. Tire impressions, due to their length, are photographed in overlapping sections, later the individual photographs may be pieced together to represent the length of the tire track.

Casting

3-dimensional impressions are cast with a material that will retain the features and physical size of the impression accurately. Dental stones (gypsum cement) is commonly used for this purpose. It is mixed with water in the proper ratio and carefully poured into the impression. After it hardens, it is labeled and collected. Coatings such as paint or hairspray are used in some impression conditions to improve the cast.

Lifting

2-dimensional impressions may occur in dust. Electrostatic lifters may be used to transfer dust impressions to a sheet of metallic backed black material for contrast and preservation. They also may be left in a variety of materials that were moist and then dried, such as water or mud. 2-dimensional impressions of many types may be lifted using gelatin lifts (gelatin material on rubber backing) or through the use of sheets of adhesive material.

Enhancement

Impressions may be enhanced using a wide variety of techniques, depending upon the substance that constitutes the impression. Oil or dried moisture impressions may be enhanced using powder brushed onto the impression. Impressions in blood are enhanced with reagents that stain or react with components of blood. Reagents that react with other substances such as minerals (iron, calcium carbonate) or proteins (amino acids) may also be used to enhance impressions, usually when the original impression is able to be transferred to the laboratory for processing, such as a dust impression on a piece of paper. Specialized forensic photography utilizing lighting and filters may also be used to enhance impressions.

These documentation and collection techniques may be applied to preserve and record other types of impression evidence, such as impressions of bare feet, gloves or other types of tracks.

This document is intended to provide general information about the collection and processing of footwear and tire impression evidence and is not intended to be a guide or to be inclusive of all possible procedures and methods for the collection of impression evidence.

References

www.theiai.org

1. Bodziak, W. J., Footwear Impression Evidence, 2nd ed., CRC Press: Boca Raton, FL, 2000.

2. Bodziak, W. J., Tire Tread and Tire Track Evidence, CRC Press: Boca Raton, FL, 2008.