Staying on the trail
The scene: you’re the first to arrive at an alleged residential burglary. The suspect’s point of entry is obvious: a smashed patio door tells you their likely point of entry. Still, apart from the missing valuables and high-end electronics, these crafty criminals have managed to leave very few signs of disturbance. No hair. No discarded property. And, most unfortunately, no fingerprints. They’re clearly experienced.
Lucky for you, it’s also been raining and a trail of muddy footprints has been left all around the house. Now it’s our turn to use this somewhat messy situation to our advantage.
Tracing those toes: casting, static charges and the “inkless pad”
There are actually several methods of obtaining footwear impressions, and these are dependent on the type of print left. If we’re looking at footprints that have left an indentation (for example in damp mud or wet cement) SOCO (Scenes of Crime Officers) can take a cast. If we’re viewing dirty footprints on a dry surface such as a wooden or laminate floor, SOCO can use fingerprint powder and paper to take an impression of the prints, in much the same way that the police do when they take fingerprints by hand. We’ll probably do this in our burgled house. Here’s an alternative method that works on a dusty surface: the SOCO can actually apply a static charge to the surface which will enable the impression to ‘leap’ to oppositely charged paper. Clever stuff indeed.
Yet the most interesting, and recent method of obtaining footwear impressions is the use of the so-called inkless pad. A special book is used: chemically treated sheets of paper large enough to hold a footwear impression are placed in one side, and a special, chemically coated “ink” slot is placed in the other. You put the sample shoe in the ink – then onto the other page and there you have it — instant footwear impressions.
Getting it on the right foot: putting it all together
Now we’ve “lifted” (as they say in the policing business) some footwear impressions, we need to take some elimination prints. Elimination prints are those taken from footwear which belongs to people with legitimate access to the building – in our case, householders. Now, before you start panicking, there’s no need to sample every shoe in the house – if our intruders have invaded in size 10 Adidas P7s for example, there’s no point taking impressions from our home owner’s size 6 rambling boots. But similar footwear will have to be eliminated. So get seizing!
The unique patterns in footwear develop over time, depending on the gait of the wearer. So if your householder and the suspect happen to have exactly the same shoe, in the same size, we still might be able to obtain a useful sample. If the shoes happen to be brand new, we might struggle to discern them from the elimination sample — which may well cause us problems in terms of evidence recovery.
Now we’ve recovered some impressions, it’s onto the police officers to perform the investigation. Perhaps a few enquiries will reveal some suspects, and allow the right people to be cuffed. Once this is process is complete, their house can be searched and, with a bit of luck, some footwear will be recovered. This will enable the recovered footwear to be compared to the samples taken earlier. If we do seize the suspect’s footwear a highly technical and detailed electronic analysis of the individual characteristics of the shoe can be performed, as mentioned above, to distinguish it from other shoes of the same type. We can also speculatively run the suspect’s footwear through a database, to see if it flags up any ‘hits’ of more offences of a similar crime type.
But, and if you’ll pardon the footwear based joke, here’s the rub: seizing a shoe doesn’t conclusively prove anything. Yes, it’s damning evidence, no doubt; but what if your suspect merely trespassed and didn’t steal anything and the real offender came before him? What if the shoes don’t belong to him? What if they’re similar, but we can’t say for certain whether they were or weren’t the shoes worn on that day? Footwear, although useful, should never be the ‘sole’ (!) forensics consideration in an investigation. Sure, it’s helpful, but far from conclusive.
Your Turn: Know of another way to trace footwear to a specific individual? Think you can help us shed light on what algorithms are utilised in computer matching software to figure out whether shoes match a particular print? Let us know in the comments; we’d love to hear from you.