Forensic Evidence

The term ‘forensic’ has evolved from a word which meant ‘of or used in a court of law’ to include the modern interpretation of ‘having to do with the use of science in the investigation of crime’ and the expression ‘forensics’ is universally used to describe the group of people and the procedures involved in the gathering and interpretation of scientific trace evidence.

This scientific evidence comes in many varied forms; shoe impressions, handwriting comparison, fibres, tool marks, pollen, anthropology, entymology, alcohol analysis, drugs analysis, paint comparisons, glass fragment comparison, DNA profiling, blood pattern analysis, pathology, tyre marks, fire investigation, voice comparison, video enhancements, identification of body fluids, firearms and ammunition comparison.

Each of these types of evidence has to be considered at a crime scene and, if present, recovered using techniques which maximise the evidential value of the exhibit. Needless to say, the first methods of recording are always photography, video and detailed note making.

Large, visible exhibits, such as knives, guns, ammunition cases, etc. are recovered, whole, into an appropriate type of packaging. These exhibits may reveal other, latent or microscopic evidence and, as such, any large item will be treated as if it has fingerprints, DNA or fibres present, and packaged carefully so as to preserve any of this potential evidence.

Similarly, clothing, furniture, the skin of a victim, walls and doors will be preserved and examined using techniques for recovering fingerprints, DNA, blood, fibres etc.

The central tenet of forensic investigation is Locard’s principle of exchange, which basically states that whenever two objects come into contact, material is transferred from each to the other. On a very simple level, this means that if a person wearing a red jumper wrestles with a person wearing a green jumper, green fibres will be transferred onto the red jumper and red fibres will be transferred onto the green jumper.

In reality, this simplistic scenario is not what we are looking for. Most textiles, even those which apparently consist of one solid colour, are actually comprised of a number of different fibre types which are often, microscopically, a totally different colour to the main fabric. This mixture can be unique to that particular type of garment, and finding all of the different fibre types, in the correct colours and in the correct proportions are what gives the ‘match’ evidential significance.

When a person smashes glass, whether it’s damaging a car window or breaking a window to burgle a house, microscopic fragments of glass are ejected from the breaking pane in both directions. As a consequence the perpetrator will have these fragments distributed on their hair and clothing.

How much glass is present depends on how close to the glass they were when the pane was broken and the mechanics of how they broke it. How much remains over the ensuing hours and days depends on the activities undertaken by the offender and the retentive qualities of their clothing. Generally, after a few days little or no glass fragments will remain.

If the person is detained, their clothing seized, and their hair combed onto a piece of paper within a reasonably short space of time after the commission of the offence, the fragments of glass will provide corroborative evidence of their presence near to the window when it was broken.

Glass from the suspect is matched to a control sample taken from the broken window pane by comparing their Refractive Indexes (R.I.)

The R.I. of one of the pieces of glass is ascertained by placing the piece of glass into a clear oil mounted on a microscope. The oil is heated, changing the R.I. of the oil. When the R.I. of the oil matches the R.I. of the piece of glass, the piece of glass effectively becomes invisible to the observer and a note is made of this temperature.

The process is repeated with the other glass fragment and, if the temperatures (and hence the R.I.s) are found to be the same, a match is made. The evidential strength of this ‘match’ will depend on the frequency with which this type of glass is encountered in the environment. In general, the older the piece of glass, the rarer it will be and therefore the greater its evidential value.

Tools used by offenders at crime scenes can, potentially, provide evidential links with the crime in a number of ways. Whenever a CSI attends a crime scene at which a tool has been used to lever open a window or door, they will recover a reference sample of the painted surface and a photograph and cast of the tool mark. They will also look to see whether paint from the tool has been deposited onto the surface and obtain a reference sample of this paint.

When a tool is used to lever a window or door, it often leaves a clear impression of the tool which gives us a clear idea of the dimensions and the type of tool used. If the tool scrapes across a hard surface, it can leave a mark which has a series of minute linear grooves, called striation marks. These striation marks are unique for each tool and the reference cast obtained at the crime scene can be fully matched to the tool.

The tool will also bear traces of the paint from the window. This paint will be chemically analysed and microscopically examined to provide another link to the crime scene. If the window is old and has been repeatedly over-painted in different colours, it can even build up a virtually unique pattern in itself.

A reference sample of the paint from the tool will also be obtained and compared with the paint deposited at the crime scene, supplying a further corroborative link.

Finally, the tool can be linked to the suspect by swabbing the handle of the tool for DNA, and searching the resultant profile on the national DNA database.

When a bullet is fired from a gun or a rifle it travels down the barrel and, as it does so, it has a spin imparted to it by spiral rifling within the bore of the weapon. This spin makes the bullet travel further and straighter and, consequently, makes the weapon more accurate.

Because of the close tolerances of the barrel and bullet, this action creates minute, linear striation marks along the sides of the bullet. These striations are unique to each weapon and, if the weapon is recovered, they can be used to compare test firings of bullets with a bullet recovered from a crime scene.

The term ‘bullets’ is sometimes mistakenly used to describe cartridges. A cartridge is the casing, gunpowder, primer and bullet (usually made of lead) which is inserted into the weapon. The bullet is fired out and the remaining casing ejected and discarded.

In order to fire the bullet the casing is struck at the rear by the firing pin of the weapon. This firing pin leaves another unique mark and finally, some weapons automatically eject the used casing as the weapon is used. This ejection mechanism also leaves a unique mark on the casing.

Blood from a wound can obviously be DNA profiled to match the suspect or the victim, but it can also be a source of information on the number of blows administered to, and position of, a victim during an assault. This blood pattern analysis can help to confirm or refute a version of events given by a defendant, victim or witness. This analysis can determine the direction in which the blood was travelling and the height from which it originated. This is particularly useful in cases where someone has been repeatedly injured and can demonstrate that someone was upright at the start of an assault and was gradually beaten to the ground.

Blood patterns on a suspect’s clothing and footwear can show that the person was in close proximity to an assault and indicate whether they kicked or punched the victim.