A specialist area of Forensic Science
Shoeprints at a Crime Scene
The officer in charge of the scene will brief the forensic examiner on the crime scene, giving clues as to what potential evidence should be special attention in the examination. The first officers on the scene will have attempted to determine the entire area of the crime, including paths of entry and exit, and areas that potentially contain evidence to link a suspect with the scene. The number of offenders is important to help eliminate shoeprints found. The officer in charge will have a list of people who have had access to the scene, and the areas they have accessed. Shoeprints to eliminate include: victims, witnesses, ambulance officers, police officers, and other people with legitimate access to the site.
Preservation of the Shoeprint Scene
The first step is to protect the integrity of the scene from contamination by people, animals, and the elements. This involves sealing off the parts of the scene where the offender has left footwear impression evidence, such as the point of entry and point of exit. If there is only one path of access to the scene, stepping plates need to be utilised to avoid inadvertent destruction of the evidence by scene examination personnel. Next, the general crime scene should be thoroughly photographedand sketched, indicating the point of entry, location and orientation of shoeprints and suspected point of exit.
Location of Shoeprints
The shoeprint scene needs to be thoroughly searched for latent prints. After a slow visual search, the most efficient way to do this is to darken the room and use an intense oblique lighting source on the areas where shoeprints may have been left. It is often the less visible imprints that leave the finest detail.
If latent prints are still not visible under the oblique lighting, especially around important areas such as the window where the offender is thought to have left the scene, the area can be searched with black gellifters. The black gellifters can be viewed with an oblique lighting source and sometimes reveal latent prints that were not previously visible.
Notes should provide information about the location and direction of all impressions as well as a brief description of each and the time of examination. These notes should be prepared so that they, along with the general crime scene photographs, can be used to document and reconstruct the scene and the relevance of the evidence.
Recovery and Enhancement
After initial photography, the optimal method of preservation needs to be established. Most footwear impressions can be enhanced to provide additional detail, but before an attempt at enhancement is made, evidence quality photographs need to be taken as enhancement techniques can destroy the impression. (see shoeprint impression photography)
There are two main categories of shoeprints, based on how deep the impression is. 3D prints are those that are commonly made in snow and dirt, and are basically a negative cast of the shoe. 3D impressions do not generally show much detail. 2D prints are flat impressions left on surfaces, and can show a surprising amount of detail.
The most common choice for preservation of a 3D impression in soil is to cast with dental stone. When set, dental stone is hard enough to clean, without loss of detail. The casting material usually comes in a zip lock bag, which can be used to mix in the water. The mixture is carefully poured next to the impression and allowed to flow quickly into the impression. The cast will harden in approximately 20 minutes, when it can be labelled, lifted, and placed where it can dry further. When the cast is sufficiently dry, it can be washed and lightly cleaned with potassium sulphate solution, to reveal the fine detail of the impression.
The other common substrate for 3D impressions is snow. There is a product that I have not used, as I live in New Zealand and snow is not common here at crime scenes, the product is sprayed onto the snow impression and hardens it. The hardened impression can then be cast in a similar manner as soil impressions, described above.
2D impressions are further categorised into two classes; positive impressions and negative impressions. These are best explained with examples: a positive impression could be made by stepping on a clean floor with dirty shoes, leaving a print like a stamp. A negative impression could be made by stepping on a dusty floor with a clean shoe, thereby removing dust from the floor where the shoe made contact.
The positive dust impression can be lifted directly with white adhesive paper if there are not too many large particles in the impression. This lifting method works by placing the adhesive paper over the impression and then smoothing over. This transfers the impression to the paper, which is peeled off and photographed. Lastly, a piece of clear acetate sheet is placed on top of the transfer to seal the impression for storage.
This method of lifting can yield excellent detail if the substrate is a clean smooth surface like a polished floor, where there should be little or no background interference.
Negative dust impressions, and interestingly impressions characterised by the absence of floor polish, can be dusted with latent print powder, and lifted with an appropriately coloured adhesive lift. Again, the negative impression needs to be photographed in situ before and after enhancement with powder, and after lifting has been completed, before the acetate sheet is put in place.
Both positive and negative dust prints can be recovered using an electrostatic lifting device (DLK). This system creates a high voltage potential across the impression, causing dust or residue particles to transfer to the underside of the lifting film. The recovered impression will be a light, dust-coloured negative outline of the outsole, on a black recovery film.
Standard chemical enhancements can be used for better contrast after photography. Examples of shoeprints that can be chemically enhanced include those made with blood, soil and oil. It is important to enhance the impression after photography, as there is a chance that detail might be lost in the process.
Analysis and comparisons
All prints collected at the scene need to be compared to any suspect prints and elimination prints. Depending on the correlation between the suspect shoe soles and the prints from the scene, the forensic scientist can reach a conclusion as to the chance of that shoe having made that print.
As with all toolmark impression comparisons, the parts of the shoes that match are grouped into a class, according to the probability of that match happening.
First are class characteristics, elegantly described in William Bodziac’s textbook as “…an intentional or unavoidable characteristic that repeats during the manufacturing process and is shared by one or more…shoes. Examples of class matches in footwear impressions include:
- outsole size
- specific shape
- dimensions
- specific design or logo
A class match is the lowest type of match and is of low evidential value by itself. However when several class characteristics are combined, like in the O.J. Simpson trial where the shoes were determined to be a rare Italian brand, the evidential value can be quite high.
The other class used to describe characteristics are identifying characteristics. Again, to quote from the Grandfather of Forensic Footwear Impression Analysis, William Bodziak, these are “…characteristics which result when something is randomly added to or taken away from a shoe outsole that either causes or contributes to making that outsole unique.”
Examples of random characteristics typically found on the outsole of a shoe include :
- a scratch
- a cut or tear
- an air bubble
- an inclusion such as a piece of glass or a stone.
Obviously it is not possible to statistically determine the probability of a given point of damage, but if the experienced examiner decides that the damage could not occur on another sole, then an identification can be declared. In this manner, only one unique characeristic is requiered to declare an identification.
Related Link: shoeprint impression photography
References
HILDERBRAND, D. S., Footwear: The Missed Evidence, Chesapeake Examiner, Jan, 1995.
BODZIAK, W, J., Forensic Footwear Evidence, Forensic Science: An Introductory Guide, CRC Press, 2003.
BVDA.com, Product Information, May, 2003, Http://www.bvda.com/eng/prdctinf/.
Adapted from my research essay for the postgrad paper "Fundamentals of forensic science" at the University of Auckland