By Dave Barclay
Footwear marks are my very favourite evidence type! Like it or not you have to stand somewhere to commit virtually every crime. And because every step leaves an impression, footwear marks can be found at virtually every crime scene – from criminal damage to murder. Footwear marks – however small or partial – are a particularly important evidence type because they can often provide definite conclusions to match the mark to a recovered shoe.
They can be successfully recovered from almost any surface, hard or soft, carpet or worktop, and can be made by transfer of materials such as mud, dust, blood or grease – so footwear also transports materials into and out of the scene, which we can also match to a source. Marks made in blood can be particularly important because they allow timing, since the mark must have been made after the blood was spilt, and the blood can be linked to the victim by DNA. They allow the type of footwear to be determined (make, model and size) by comparison with reference collections of current sole patterns. This is a speedy process and can provide rapid crime intelligence, giving the type of shoes worn by a suspect, and the number of suspects present at the scene.
Footwear marks also have the potential to identify each shoeprint absolutely and thus provide conclusive evidence to associate it with the crime. This is possible because soles become damaged with nicks and small scratches during wear, and therefore leave unique prints. The composition of many cheaper shoes means that hundreds of tiny air bubbles are locked into the sole when it is formed; these become visible in a random pattern as it wears. Local collections of marks found at crime scenes can be used to link scenes together. An initial classification and sift can be carried out rapidly in force, using commercial software, with further detailed comparison often following at a specialist laboratory.
Although they are always present, footwear marks may not be obvious and so it is important to think through the crime and pay particular attention to the point of entry/exit or the actual point of crime in offences such as robbery or assault. CSIs try to visualize where firm foot contact must have been made: for example, where someone has climbed in through a window onto a worktop, or stood in front of a drawer to take out a knife.
Footwear marks are usually revealed by a close inspection with powerful directional light, or even UV or laser. The most obvious impressions, however, may not be the best for comparison purposes – think of making potato cuts with poster paint – the best impressions are usually the third or fourth ones after most of the paint has gone and it’s the same for mud or water. CSIs have special techniques available to enhance faint marks, and marks in dust (electrostatic lifting), mud and sand (powder or casting), blood (protein chemical staining) and grease (powder or chemical treatments) can all be enhanced successfully.
At the scene, the marks themselves can be either photographed or lifted using electrostatic attraction, adhesive tape or gels. But if possible, the item on which the mark has been made should be recovered for subsequent analysis back in the CSI lab. Of course the shoes may bring materials into the scene such as soil vegetation and pollen and will take materials out – such as blood or fibres from carpets. All these can be used to link the shoes when recovered to the crime, and we can tell who shoes they are by swabbing inside the shoes for DNA.
One other area can also be important to the assessment of a crime is tracing the pattern of marks around the scene, and establishing which came first, can clarify who did what to whom, and often disproves the story of a supposed witness.
Often a criminal will dispose of clothing that they can see is contaminated with blood, but they are much more reluctant to get rid of shoes. A good job too, as can be seen from the photograph which shows a tiny speck of blood, which went unnoticed by the suspect, but which could link him to the scene and the victim.
Despite all of this, footwear mark evidence is still under-regarded, as it was when Conan Doyle was writing his Sherlock Holmes investigations 120 years ago: ‘There is no branch of detective science,’ says Holmes, ‘That is so important and so much neglected as the art of tracing footsteps.’
(A.C. Doyle: A Study in Scarlet 1887)