‘Criminals can’t fly’ say forensic scientists. What they mean from this rather obvious statement is that people committing a crime have to put their feet down somewhere so there is always a chance of finding an impression made by their footwear.
The skills required of a Scenes of Crime Officer include determining both the paths of criminals to and from the crime scene and whereabouts they have been at the scene – and then carefully searching for footwear marks. The best way of recovering marks is simply to take items bearing them back to the laboratory if possible. However this often cannot be done in which case photographs are taken. Special lighting conditions and optical filters are sometimes used to produce the best photographic contrast. Impressed marks can be recorded by making plaster casts although their use is not as frequent as some texts might suggest. Other specialised techniques can be used such as gel lifts for dusty marks on a smooth surface and electrostatic lifting of dusty marks on a carpet.
Test marks are made from the shoes of suspects using ink, powder or grease and are then compared with the scene marks. As well as the pattern and pattern size, forensic scientists look for general wear features on the shoe and specific random features such as cuts. Some appreciation of biomechanics is also required because the manner in which weight is transferred to a footwear item when walking or running affects the wear pattern. Overpronation occurs when there is excessive inward rolling of the foot; oversupination when there is excessive outward rolling.
The interpretation of footwear marks that match items of footwear requires extensive knowledge of manufacturing processes of the footwear along with the many ways that wear can alter the pattern of the footwear undersurface. Training shoes are particularly important in these respects as they are encountered very often. The way in which the undersurfaces are manufactured using injection moulding needs to be understood as well as the extent to which different moulds are used for different shoe sizes and the nature of pattern artefacts that can be introduced by the moulding process.
Footwear impressions at crime scenes often show only a relatively small proportion of the footwear undersurface. But because unusual features of wear and damage may be identified on the smallest of impressions – potentially providing very strong evidence – they are always worth taking.
As with many other areas of forensic science, computerised data bases are used to assist interpretations. In the United Kingdom a system called Footwear Information Technology (FIT) has been used in a variety of ways including comparing questioned scene marks with footwear of people previously convicted of crime, estimating how common or rare a particular pattern is and identifying the make of a shoe from a pattern.
In our events students make test marks from training shoes using a modern methodology, compare impressions with scene marks and make interpretations – all in the context of a wider casework investigation.