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Basic principles – preserve

Basic principles - preserve
You will probably have realised that there is a blurring between control and preserve. A well controlled scene is a well preserved scene. But what constitutes well controlled depends on the circumstances of the offence and location. The main determining factors are in two pairs. Is the offence a major crime or a volume crime? Is the scene outdoors or indoors? There is no absolute definition of a major crime, but a useful guide is that it will be one that is tried in a higher court before a jury in most Western jurisdictions with a more severe penalty, for example a prison sentence of more than two years, and involves the most serious crimes against the person such as murder and rape.
There are many lesser crimes, but the distinction we are making here is that of volume crimes, which are exactly what the name implies, simple assault, house breaking, and so on that make up the bulk of reported incidents. Although these are lesser in recent years, they have been the focus of attention because DNA databases have revealed that the perpetrators of major crimes are often also the perpetrators of volume crimes. The graphic shows the number of United States cases where the investigation has been assisted by information in their national DNA database.
We will address the issues surrounding DNA databases But for now, note that while the FBI data gives a good indication of the size of the activity, it is very loose in regards to its impact with no discussion on what is meant by investigations assisted.
Indoor scenes tend to be easier to control and preserve than outdoor ones. The walls and doors can provide a ready made cordon and access control. The integrity of the scene is protected against most environmental influences. This is not an absolute, however. It assumes that we’re dealing with private property or smaller, discrete business premises. It also assumes that there are no substantial grounds that were involved in the crime, for example access and egress or disposal of tools or weapons and no damage or malfunctioning of things like water supply, heating and cooling, and lighting. The image is a finished house in a woodland setting with no fence or gate. What are the possible implications of this for control and preserve?
Outdoor scenes are much more difficult to control and preserve. Think about the scene in this case. Where would you place the cordon? Why? What about the weather, and what about traffic on the road? Finally, some people include a third type of scene, namely vehicles. These are usually associated with the outdoors, but to some extent cocoon the place of interest protecting it against environmental degradation. Again refer to the actions in the case study.
Preserve the scene from contamination. One element that often features in fictional crime scene stories but is factually correct is the use of protective suits. Usually white in colour and made from Tyvek, a strong, breathable polyethylene fibre, and worn with nitrile examination gloves, face masks, and footwear covers, these prevent any transfer of traces such as fibres and DNA between examiners and scene and between the scene and the examiners. These would be typically worn by teams at the scene of major crimes. In contrast, examination of a volume crime scene would be conducted by one or at most two officers with minimal protective clothing, often only gloves and face masks.
Preserve the scene from the environment. The image shows a team completing their notes after the examination of a scene that was protected against the weather by erecting a portable tent. The tent also protects the scene from intrusion by busy bodies. The photograph illustrates the arbitrariness of classification as indoors or outdoors. The tent is protecting an outdoor area associated with the investigation of a house. Preserve physical evidence. The definition of real evidence was constructed around arising from material objects. These objects are often referred to as physical evidence and can be anything from a trace invisible to the naked eye to a large structure or manufactured object.
Examples include minute textile fibres transferred between the outer clothing of a rapist and the underclothing of the victim, to provide evidence of an intimate contact, and examination of the damaged wheel and deflated tyre of the vehicle involved in a fatal collision, to provide evidence as to whether the collision was caused by an accidental failure of the tyre or the damage was a result of the collision. Whether large or small, the evidence must be protected usually by appropriate collection procedures and secure packaging. Secure means that the package is sealed in a way that is tamper evident and prevents loss to and contamination from the external environment during transport and storage to the laboratory. This can be quite simple.
The illustration shows paint tins used to collect and preserve debris from a fire scene. This must be placed in a container that does not permit any loss of volatile accelerant such as petrol that may have been used in an arson. The paint tins do this well. Note also the sealing tape over the lid. This is special evidence tape that will break if there is any attempt to remove it. Note also there are two pieces of tape, and it is not possible to open the lid without breaking the seal. Collection and packaging. Trace materials invisible to the naked eye are protected by the manner in which the bulk item on which they are possibly located is handled.
Items such as clothing are carefully placed into evidence bags and sealed. Items from scene, from victim, and from suspect are all collected and packaged separately to prevent any possibility of cross contamination. One of the best ways to collect the evidence in the laboratory is by taping. The surface is dabbed with the sticky side of clear adhesive tape, Scotch tape or Sellotape for example, and then fixed to a glass slide to be examined by microscopy. The same technique can be used at the scene to deal with recovery of traces from larger solid objects that cannot be packaged.
Looking at the physical evidence in the murder by the loch, in some circumstances the investigators may have chosen to record tyre or footwear impressions from around the car to try and find information about the assailant’s vehicle and footwear for later comparisons. The impressions can be preserved by photography and or casting depending on whether they are two or three dimensional. Spent ammunition is collected in a way that avoids leaving any marks, DNA, or fingerprints, for example, by picking up while wearing gloves and placing in plastic jar or cardboard box. And fingerprints can be collected by a range of methods from dusting with powders to specialised chemical treatments.
The illustration shows the sole of a worn trainer, the three dimensional imprint it made when the wearer trod in soft ground and to the two dimensional imprint left when the wearer kicked a door. The same basic principles apply to biological evidence, but with the added complication of making sure that there is no biological deterioration during transport or storage. This means drying, whether the evidence is a vaginal swab in a rape, blood stained clothing in an assault, or cannabis plants in a drugs case. If packed in plastic, these will all undergo microbial and fungal degradation, go mouldy in other words, just like salad greens or cheese in a plastic bag even if kept in the fridge.
Crime scene units in forensic biology laboratories have special drying areas where the items can be laid out, physically separated from each other, and dried in a gentle air flow. A modern alternative is to use breathable evidence bags. As described for the Tyvek scene suits, these are made from spun polyethylene fibre. Some chemical evidence such as drugs, reagents from clandestine laboratories set up to make bombs or drugs, and accelerant residues from fire scenes require special handling. These scenes will usually be examined by specially trained chemists who will have the equipment needed to collect and preserve the evidence. We will close the section on evidence integrity with a brief look at its integrity as viewed by the law.
Courts universally require to be satisfied as to the authenticity of the items collected and tested before they will accept them, and therefore, the derived evidence as exhibits in a trial. This is achieved by the chain of custody, which begins with the evidence log created during the examination of the scene. Every item must have a record as to what it is, where it was found, who has handled it in any way, when these things took place, and where it has been between collection and production in court. The chain of custody is an unbroken chain authenticated by the signatures of those handling the items and the date when and place where it was in their possession.
The signatures may be written or electronic. The breathable evidence bag previously shown has a chain of custody record template printed on it.

What is it that we are going to preserve, and how will we do this?

Part of the answer is that there is a blurring of control and preserve, and what happens depends on the crime and the scene.

The first is discussed from the perspective of major crime or volume crime. The major crime part is easy – murder, rape, bombings, all fall naturally into this category, and the response is also easy, with a full deployment of resources to prevent changes to the scene such as loss or contamination. Volume crime includes house-breaking and has often been treated as minor and not worth bothering about. However, when it was discovered that DNA databases were producing links between minor crimes and major ones, the responsiveness to volume crime became more critical in order to preserve vital DNA evidence.

The video discusses the use of cordons to control scenes and prevent their degradation, and shows how this is simpler in well-contained indoor scenes than in less well-defined outdoor ones.

The main part of the video deals with the specifics of preservation against contamination and the environment, and the preservation of the different kinds of materials identified and collected from a scene.

These are all about preservation of the physical integrity of the evidence, and the final preservation element considered is legal integrity, or chain of custody.

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Introduction to Forensic Science

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