In the first article in this series, ‘The Golden Hour’ we looked at the crucial steps to take within the first hour after finding a body. Then the Senior Investigating Officer (SIO) and Crime Scene Manager (CSM) got decked out in their PPE and we made sure we had our forensics kit in order, in ‘The Kit’.
Now let’s start collecting that evidence!
In this article we will look at the principles of evidence collection, methods and patterns for searching.
Good Evidence Collection: A Few Principles
If you pick up and book on forensics I can guarantee with the first five pages you will come across the phrase ‘Locard’s Exchange Principle’.
What Locard is saying here is that wherever you touch a surface be it the floor, wall, table, cup or someone’s arm: you will leave behind a part of you.
Think about it – even if I walked into a mate’s house right now and didn’t touch anything, I have still left shoeprints from my trainers in the carpet (possibly bringing in dirt too). Oh, and some of my cat’s hair from my jeans has dropped onto the floor as I shut the door on my way out.
In general, the closer a piece of evidence is found to the body at a scene, the more important it is. That’s because, the further you find evidence from the body, the more easily the defence can claim it was left by chance or coincidence. It goes without saying therefore, that the more uncommon an item is, the more important: Uncommon items can be more easily linked to a suspect, e.g. pet hair, rare fibres from clothing.
One of the key things to remember is that the purpose of good evidence collection is to make a strong, scientifically sound case against the prosecution once they are caught. And that bring us to our second key principle: chain of custody.
‘Chain of custody’
This refers to the documented paper trail of all evidence from the point at which it is seized, through custody, control, transfer, analysis and disposition.
We must ensure that from start to finish, the life of a piece of evidence, or an ‘exhibit’ is handled according to strict procedure. At this point in the case, we are most concerned with the handling and labelling of each exhibit.
- Every item found must be individually packaged in a suitable container (See Crime Scene Series: The Kit for more details)
- Packaging is tamper-proof, so that it will be obvious if an item has been accessed without permission
- Labels should include time, date, scene location/number, handler, a brief description of the item and sometimes the location of the item to key reference points at the scene
So what evidence are we looking for at our scene? First and foremost we are looking to retrieve those details that are most transient or fragile: those that may not last long or could easily be broken. This could include recording temperature, and things you can smell at the scene.
In future articles I will go into further detail about the categories of evidence we are looking for:
- Impressions (fingerprints, shoes, tyres and tool marks)
- Exchange materials (hair, fibres)
Crime Scene Examiners (CSEs) (sometimes called Scenes of Crime Officers/SOCOs or CSIs) will identify and collect evidence, with the instruction of the CSM, but do not carry out the analysis of the evidence. Nearly 45% of all registered forensic practitioners are CSEs, demonstrating that this is increasingly a specialised role; especially considering this job used to be carried out by police officers. This need for specialist skill in evidence collection strengthens the validity of the case brought to court (and again, chain of custody).
Once evidence is identified, collected and labelled, it is handed to the Exhibits Officer assigned to the investigation.
In order to ensure every square inch of a scene is thoroughly checked, geometric patterns are usually established in order to adopt an organised approach to evidence collection. Each pattern has its own benefits and drawbacks; the pattern we can use depends on several factors:
- number of CSEs available
- size of scene
- time of day
- timeframe available (e.g. a quick pattern would be needed for a kidnapping case as it is time-sensitive)
I have made a handy infographic for you to keep as a reference at the end!
Spiral or radial search
Used one of two ways, searches can either start from the body and move outwards in a spiral or move from the edges of the room and move in a spiral toward the body. It is usually used to locate a specific object in relation to another, e.g. a bullet casing near a gun.
This is a rarely used pattern given its limited use. It only permits one CSE to ensure the spiral pattern is kept consistent as it radiates in or out; having just one CSE working on the scene is very time consuming.
Quadrant or zone search
The scene is divided into zones, perhaps then divided further, dependant on the amount of evidence thought to fall in each zone. This approach is generally used in larger scenes. ‘Zones’ may be based on elevation, particularly at scenes involving blood splatter on walls or bullet holes; zones may be divided into ground, mid-level and head height, for example.
The benefits of this search pattern are that each CSE can be given their own zone, so that many CSEs are working on the scene at once. It is also particularly useful for scenes that are already loosely divided into zones, such as rooms within a house.
Grid, boxed or double strip search
For a grid search the scene is covered in a criss-cross motion, so that the same area is covered twice but from different directions. This could be carried out by one or two CSEs at the same time.
This is the most thorough search pattern as the area is covered by different people looking at the same objects from different angles. It is also the most time-consuming.
Linear search may also be referred to as lane, line, link or strip search. CSEs organise into a line and move together in one direction in order to cover the entire scene. This pattern is generally used for outdoor scenes and searches for larger objects.
This may be done on hands and knees at a smaller scene, sometimes referred to as a ‘fingertip search’. For larger scenes this may be done on foot, and with a larger gap between the CSEs, sometimes with lanes marked by tape.
This kind of search can also include volunteers if the scene is outside and covers a very large area, though volunteers require upfront training so they do not attempt to move evidence.
Wheel, ray or star search
Starting at the body, the search is done in straight lines out from the body and back again.
The obvious drawback for this type of search is that the gaps in between the ‘spokes’ of the wheel are potentially missed. It also requires several CSEs to be involved.
Although we now know the principles behind evidence collection, our CSM and SIO cannot agree a strategy for the scene without first photographing and sketching it; this is a process which is far more in-depth than you might think!
Stay tuned for Crime Scene Series: Sketches and Photography coming soon!
Over to you
What did you think of this article? Are there aspects you would like to know more about? Let me know in the comments!