How do police develop, and collect, footprint and tire track impressions – and how do they perform forensic examinations on the impressions?
Much like fingerprint impressions, footwear and tire impressions can be deposited on just about any surface; and just like fingerprint impressions, they fall into three basic categories. Visible (patent), latent, and plastic.
Impressions that are categorized as visible, are impressions that can be seen without powders or chemical processing. A visible impression is simply the result of the transfer of something from the shoe or tire surface, onto another surface that can be seen without aid. Bloody footprints are a common example; blood is transferred from the shoe sole onto a surface, say a tile floor. For instance, the person leavening the impression stepped into wet blood, and by walking, left the impression. Tire impressions of this type most commonly are found on concrete or asphalt, and they are impressions made by road dirt, or mud that was left on the surface when deposited there from the tire tread area. These impressions don’t require processing to be viewed.
A latent impression is one that is not visible to the unaided eye. These impressions are made up of dust particles that have been transferred to a surface via static charges, or by just about anything on a shoe sole that transfers ,unseen, onto a surface, like wax. Fingerprint powders, or chemicals are used to develop the impressions. Common examples are shoe impressions left on a tile or hardwood floor, tire impressions left on smooth concrete, like a garage floor.
The third type, the plastic impression, is an impression left in a soft surface. Examples include, shoe or tire impressions left in mud, soft sand, or snow. These impressions are visible by the unaided eye, and don’t require processing to be visible.
Once located, any impression must be documented, photographed, and collected when possible.
For visible impressions and processed latent impressions, they will be photographed from above the impression, at a 90 degree angle, with and without a measuring device. Usually, oblique lighting is used to create shadows on the impressions when photographing them; the shadows help show details in the impressions. These impressions can then sometimes be lifted with fingerprint lifting tape, acrylic adhesive lifters, or gel lifters. (Gel lifters use latex and a low-tack adhesive gelatin layer to collect the print, acrylic adhesive lifters use a sticky acrylic film with a mounting back to collect the print). For impressions consisting of dust particles, electrostatic lifters can be used, one positively charged section, and one negatively charged section. Additionally, there are kits that use a metalized lifting medium that will be charged with an electric current, which attracts the particles to be lifted.
For plastic impressions, casting is the most commonly used collection method. These impressions will be photographed the same way the other impressions are, then casting materials, such as dental stone, will be poured into the impression, and allowed to dry. The cast is removed once dry, and collected. A properly done cast gives a three-dimensional model of the impression. (Good impressions are hard to obtain in mud, and dirt, because of grass and debris. I have limited experience with snow, due to my location in Texas, but it makes a great impression if the snow is deep enough to not include grass)
Where a suspect or suspect vehicle is known, samples are taken from shoes or vehicle tires so they can be compared to the evidentiary impressions left at the crime scene; in major cases, the shoes and/or vehicle will be seized as evidence. Minor cases, photos and inked impressions of shoe patterns or tire tread patterns can be taken for visual comparison.
An examiner looks at two types of characteristics when examining tire or shoe patterns.
One is Class Characteristics. Here, the analysts looks at say, the collected shoe patterns to determine what brand of shoe the suspect wore. In this case, the examiner would only be examining the evidence in an attempt to identify the type and brand of the shoe/tire, to help investigators narrow down a suspect or suspect vehicle. The goal is to determine the brand, make, etc, and not to find individualized points of comparison in the impression. Examiners can access databases that have examples of shoe soles, and tire treads to determine the brand/model of a shoe or tire. Further, the FBI maintains a database that contains information and images of specific shoe sole patterns, and tire tread patterns. Class characteristics narrow down the search from every shoe in the world, to a specific brand of shoe.
The second type of characteristics are called Individual Characteristics. These are used in comparing a suspect’s shoe or tire to the impression left at the scene, in a side by side comparison. Individual characteristic examinations look at individual, specific, unique characteristics of the shoe sole or tire tread. (Of course, Class Characteristics would have to be in agreement here, or there would be no Individual Characteristic examination performed) Characteristics examined in the Individual Characteristic category would be unique damage, nicks, and wear patterns on the contributing shoe or tire. In this case, the examiner will perform a side by side comparison of two impressions, with the examiner looking for quality and quantity of the shared unique features between the two items being compared. The comparison can yield these results: Inclusion (a match between the two impressions; Exclusion (no match); or there could be an inconclusive finding between the two impressions (can’t match, but can’t exclude).
Like in fingerprint examinations, all inclusions but be verified by an independent examiner who has the same findings.
The Scientific Working Group on Shoeprint and Tire Tread Evidence (SWGTREAD) has written standards outlining minimum qualifications and training for footwear/tire track examiners, and they have documented suggested best practices as well.