Chapter 1: Recovering Evidence at the Crime Scene
You should be able to:
Explain the meaning of ESLA and state the type of evidence that can be recovered with it
For two types of evidence, state how they might be recovered from a scene of crime
Explain the meaning of latent when the word is applied to a fingerpint at a crime scene
State Locard's Law and say why you think TWO-WAY transfer of evidence is better than ONE-WAY transfer
Part 6) Recovering Evidence at the Crime Scene:
By now, the CSI will have searched the scene and identified and marked-up physical evidence that will need to be collected. If the evidence has not already been photographically recorded, photographs will be taken just before recovery and packaging takes place. In fact, this is usually what actually happens especially at volume crime scenes such as at burglaries.
The CSI will recover each different type of evidence using a method that prevents the items from becoming contaminated or damaged. Hairs and fibres, for example may be recovered either by using a pair of tweezers (if the fibres are easily visible), or by using soft-adhesive sticky tape in what is called a fibre-lift. Latent fingerprints will first be revealed by dusting them with one of a range of powders and brushes, photographed, and then "lifted" away from the surface again using a soft-adhesive sticky tape. And footwear marks may be recovered using an electrostatic technique (called ESLA) which is able to reveal even only traces of dust deposited by a shoe on a floor or door surface. Often, different types of packaging can be used for the protection of the same type of evidence - there isn't actually any really hard and fast rule. The important point is that the CSI must use the best (the most appropriate) method he or she thinks for recovering the evidence to maximise its value when it is later examined in a forensic laboratory. And when in doubt, they will ask a more senior and experienced CSI for advice.
A Reminder about Locard's Law
Put simply, Locard's Law tells us that "every contact leaves a trace." Let's think about this in a little more detail.
When evidence is deposited at the scene by the person who carried out the crime, a ONE-WAY transfer has occurred. A fingerprint left by the perpetrator is an example of one-way transfer, and it has the potential to link the person who carried out the crime to the crime scene. But remember, the person who carries out the crime might not only leave bits of themselves behind at the scene, they might also take trace materials away from the scene on themselves and their clothes. If the person who left his fingerprint also broke a window to gain entry to the property, small particles of glass will have been transferred to, and will remain on his clothes long after the crime took place. Forensic scientists call this a TWO-WAY transfer, and such transfer of evidence provides much stronger evidential support placing the suspect at the scene of crime. Whereas traces of evidence can only be recovered from the person who has carried out the crime after they have been arrested, the CSI will recover physical evidence from the scene they have attended.
Locard's "law" or "Exchange Principle" tells us that when a crime is committed, physical evidence will be left at the scene of crime, and the perpetrator will take traces of evidence away from the scene of crime.
We see below someone attempting to force entry to a shop till. If they have used a tool such as a screwdriver, it is likely that a toolmark will be made on the till. The CSI will be able to make a cast of the toolmark, and if the tool is recovered, the toolmark can be "matched" to the tool. Footwear marks will also be left at the scene, and these can be recovered by the CSI. If shoes from a suspect are recovered, these can be "matched" to the footwear mark.
If the store used "Smart Water", the person who carried out the crime will unknowingly have been "tagged" with a fluorescent dye when they committed the crime. This evidence can be used to uniquely place that person at the crime scene well after they have left the scene.