For NOVEMBER 2000 issue

SCIENCE IN CRIME DETECTION

Tire Imprints at the Crime Scenes

-Dr. Anil Aggrawal

In today’s mobile world a significantly large percent of all violent and serious crimes involve the use of a vehicle. Most criminals, out of habit and necessity, use vehicles to travel to and from the scenes of the crime they commit. In many of those instances, their vehicles travel over surfaces, which retain the impressions of one or more of their tires.

The awareness and importance of these impressions must be in the minds of the first officers who arrive at the crime scene. Reasonable priority must be given to searching for and preserving fragile tire impressions and other evidence which might otherwise be inadvertently destroyed.

These tire impressions are left in a variety of places, including road shoulders, dirt and off-road surfaces, on bodies, clothing and surfaces after a vehicle drives through materials like blood, grease or oil which then are re-deposited on concrete or asphalt roads. The complex design and the changes brought about by the wear and tear of the tire quickly make each tire unique. These impressions are of great importance, for they can offer proof that a particular vehicle was at the scene of the crime and in many instances, was actually involved in the crime. Law enforcement officers to help solve crime cases, routinely use this valuable form of physical evidence to make positive identification of a known tire impression.

In the majority of cases, a forensic examination of a tire impression is conducted in an attempt to associate a specific tire with a tire impression from the crime scene. This is done in an attempt to prove the suspect vehicle was at the scene. An examination of this type involves a comparison of the tire evidence with the tires from the suspect vehicle and takes into account the tread design, tread dimension, and wear characteristics and random accidental characteristics. Examination results range from associations of the class characteristics of design and dimension to absolute positive identification or elimination of the suspect tires.

In some instances, there will be more than one tire design represented at the crime scene. This can include a vehicle equipped with two, three or even four different tire designs. If the suspect vehicle has the same respective designs, this factor becomes increasingly significant, particularly if none of the tire designs are the original tires which were on the vehicle when it was sold and also if the positions of the tires on the vehicle correspond with evidence of those positions from the crime scene.

Consideration is also made of other factors such as tire compatibility with the vehicle, tire size mixtures, unusual wear factors, retread source information, double axle tire positions, tire studs etc.

General Crime Scene Photography: What is used? How and Why?

General crime scene photographs are sequential photographs taken from long, mid-range and close distances to the evidence with a “zoom-in” effect. This allows for photographic documentation of the items of evidence, and the relationship of that evidence to its surrounding. The “general scene” photographs are not intended for comparison of the fine detail in a forensic examination of the evidence but primarily to the crime scene.

The camera used for the general crime scene photographs should be capable of taking the full range of photographs. A 35mm of 2 ¼ inch negative format camera, equipped with a zoom lens is suitable. Color film should be used since it gives a more natural representation of the crime scene. Color films having an ISO of 200 or 400 are generally used since they are more flexible under a variety of lighting conditions.

A “Photographic Log” should be taken as a supplement to the written notes as a means of recording what the various photographs depict.

Examination of Quality Photographs: Why it is so vital to tire impressions

“Examination quality photographs” are those which are taken in a prescribed manner specifically to record the fine detail in the crime scene impressions for a forensic examination. For tire impressions, close-up “examination quality” photographs should always be taken. Examination quality photographs involves the use of a quality camera and macro lens capable of manual focus, positioned over the impressions on a tripod, using fine-grained color or black and white film, as finely-divided scale placed along side of and on the same plans as the tire impression and specialized oblique lighting to increase the contrast and visibility of the impression. The use of the finally divided scale is important for it allows the photographs to be accurately enlarged to a natural size so they can, in turn, be forensically compared against the tires from the suspect vehicle.

The Recovery of Tire Impressions from the Crime Scene.

Because tire impressions are larger than shoe impressions and often very long, they offer a different set of challenges and requirements for their successful recovery. Their proper recovery, however, is reasonably easy with minimal preparation and understanding of the process.

When a tire rolls over a surface, often there is a direct physical transfer of its tread design and size features onto or into that surface. The degree of detail retained by the surface varies and is dependent of a number of factors. It is important to employ techniques that will successfully be able to “recover” the maximum detail, which has been retained in that impression. This is a most “critical” step, for if the maximum amount of detail is not retained, the subsequent examination will be adversely affected.

The recovery of tire impression evidence from a crime includes (1) written documentation such as sketches and notes (2) general crime scene photographs and (3) examination quality photographs. In addition, if the impression is three-dimensional impression, it should be cast with dental stone.

Written Documentation: Additional Tips.

One method commonly used to relate the written notes, photographs and cast impressions to one another is to assign a number or letter to each impression. For instance, at the crime scene, by placing a numbered cone or marker beside each impression, those numbers will be recorded in the general scene photographs. The notes, examination quality photographs and casts of those impressions should also utilize that same number.

Notes and if necessary, a sketch (supplemented with general scene photographs) should be taken to record any information which can be observed and which later may be of use concerning those tracks including:

1. A general description of the tire track as observed at the scene.

2. The direction of the vehicle’s travel, if known.

3. A sketch showing which tracks correspond with the left front, right front, left rear and right rear positions of the vehicle, if known.

Other factors, such as the soil and weather conditions (wet sand, new snow, raining, etc.) should be noted. A description of the area the tire impressions are in (shoulder of road, dirt form road, etc.) and any information on observations which would relate the tire impressions to the event of the crime (impressions tracked through pool of blood).

Captions to figures

Figure 010. Photograph of Goodyear Vector Tire impression

Figure 011. Tire casting 2 from multiple homicide case

Figure 012. Tire casting 3 from multiple homicide case

Figure 014. Tire casting 7 from multiple homicide case

Figure 016. Criterion steel-belted radial

Figure 021. Photograph of sandy tire imprint in one criminal case, known as Monterey case

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(Dr. Anil Aggrawal is a professor of Forensic Medicine at the Maulana Azad Medical College, New Delhi-110002. His E-mail is dr_anil@hotmail.com and his Website address is: http://members.tripod.com/~Prof_Anil_Aggrawal/index.html

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Note to the Editor: Please make all correspondence at S-299 Greater Kailash-1, New Delhi-110048. Phones (R): 6465460, 6291942, 6413101; (O) 3239271-4

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