Shoes create impressions at the scene of a crime called shoeprints and can be extremely informative to the forensic investigator. The sole of a shoe picks up various kinds of material as a person walks, and this is readily transferred to other surfaces, creating an impression that can reveal the pattern on the sole. Investigators look at soil, particularly around the potential entry and exit points of a crime scene, as well as carpet, linoleum, paper, and dust to try to detect shoeprints. If a shoeprint is found in a pool of blood , it can serve as incriminating evidence .
There are three kinds of shoeprints: patent, plastic, and latent. Patent shoeprints are clearly visible and come from tracking through a substance like paint or dirt and leaving some behind each time a step is taken. A plastic shoeprint occurs when a shoe sinks into a soft substance like snow or mud. Latent shoeprints are those that are not visible to the naked eye and often occur on a hard surface like glass or concrete. The techniques used for collecting shoeprints vary, but include dusting with special powders, electrostatic lifting, and making plaster casts. A photographic record is always taken as well.
Back in the laboratory, the forensic investigator will determine various characteristics of any shoe-prints collected. The pattern itself can be linked to specific manufacturers and shoe types. Very few shoe soles are made of plain leather alone, and the patterns on a pair of trainers can be complex. The investigator will probably have access to a shoeprint database to identify the origin of a shoe. It may also be possible to determine the size of the shoe from the size of a sole, and this can help build a physical profile of a perpetrator.
Each individual has their own way of walking, which has an impact on the way their shoes wear down, and this will be evident in the shoeprint. It may be possible to determine if the perpetrator had a foot deformity or a limp from the way their shoes have worn down. As someone walks, the soles of their shoes also acquire a unique pattern of damage consisting of tiny cuts, scratches, and abrasions. Because no two people ever tread exactly the same route over a period of time, this damage pattern is unique to each shoe sole and can be powerful individualizing evidence.
To compare a shoeprint found at the scene of a crime with that from a suspect's shoe, the investigator has to create a print from the latter. One way is to coat the shoe sole with a light oil by pressing it into foam rubber impregnated with oil. The shoe is then pressed onto paper, creating an oily print that can be visualized with magnetic powder. If a plastic print is needed for comparison, the shoe will be pressed into a similar surface to the one in which the shoeprint was found. It is important to try to reproduce the mechanism by which the original shoeprint was made in investigating a suspect's shoeprint. The argument that both came from the same source—the suspect's shoe—then becomes much more convincing.
see also Impression evidence.