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Note: If you have completed the rest of the lessons in this series, you may wish to skip this information as it is repeated from earlier lessons.

The work of a forensic scientist falls into three broad groups.

1. Field (collecting the evidence)

Forensic scientists can find evidence from the crime’s perpetrator at almost every crime scene. Trace (or fragmentary) evidence is any type of material left at or taken from a crime scene, or the result of contact between two surfaces such as shoes and the floor covering or soil.

A team of specialised police technicians collect trace evidence from the scene. This team goes to the scene of the crime first and seal it off. They wear specialised protective clothing to avoid contaminating any evidence. They then video record and take photographs of the crime scene, including the victim (if there is one) and items of evidence. Any evidence found is put into its own sterilised containers (often a sealable plastic bag) and labelled for later analysis at the laboratory.

Some of the things needed by police technicians in the field include:

Camera: to record the scene, with a photo documentation kit marking evidence
Torch
A sexual assault evidence kit: used to collect evidence in rape or assault cases, one each for the victim and the suspect. The kit includes medical history forms, clothing collection bags, containers for debris such as nail scrapings, slides and boxes for swab collection, paper disks for saliva samples, and blood vials.
Tape: to seal off the crime scene area (blue and white police tape)
Disposable protective clothing, masks and gloves
Notepad and pens: to record observations
Entomology kit: for collecting and preserving insect evidence. This kit includes gloves, jars, ethyl alcohol, labels, scalpel, spatula, and tweezers
Paper, plastic bags and glass tubes: for collecting evidence
Cast kit: with putty and frames for making casts of tyres, footwear, and tool mark impressions
Forceps and cotton swabs: for collecting evidence such as hair and fibres
Labels for evidence
Hazmat kit: for handling hazardous materials

2. Laboratory (analysing the evidence)

Forensic scientists visually examine the evidence arriving at the laboratory from the crime scene. Notes, sketches, photographs and casts of any impressions are made and recorded. The scientist then subjects the evidence to any relevant tests. These tests include:

Body cells and fluid

Blood and other bodily fluids, like saliva and skin cells left at a crime scene, can provide valuable information. They can be used to identify who was at the crime scene and what might have happened. They may also provide DNA evidence to analyse (explored further in the lesson Cracking The Case With The Code).

Even when the weather or perpetrators wash away evidence, field-based forensic scientists can detect traces of blood and other bodily fluids. Crime-scene investigators can treat areas they believe to contain traces of blood or other bodily fluids with a fluorescent chemical that is visible under ultraviolet light.

Where a forensic team finds what they believe is blood, reagent tests, known as presumptive tests (such as the Kastle-Meyer test used in this lesson), are used to confirm the presence of blood. In the Kastle-Meyer test, the indicator used is phenolphthalein, which will produce a pink colour if reacting with haemoglobin in the blood. Further tests at the crime lab can determine if it is human blood or not. Scientists will use the blood to determine the blood type as well as DNA fingerprinting.

Types of bloodstains include:

Passive bloodstains: patterns created from the force of gravity (e.g., a drop, series of drops, flow patterns, blood pools, etc.)
Projected bloodstains: patterns that occur when force is applied to the source of the blood (e.g. low, medium or high impact spatters, cast-off, arterial spurting, expiratory blood blown out of the nose, mouth, or wound).
Transfer or contact bloodstains: patterns created when a wet, bloody object comes in contact with a target surface may be used to identify an object or body part (e.g., a piece of fabric a bloody knife has been wiped clean on).

For further information, please visit the Australian Police - Forensic Serology.

Fingerprints

Dactyloscopy is the study of fingerprint identification, and police investigators are experts in collecting ‘dactylograms’, otherwise known as fingerprints. We can use fingerprints to identify who was present at a crime scene as no two people have the same fingerprint pattern. These patterns remain unchanged for the life of an individual unless the individual acquires permanent scars or suffers a skin disease. Scientists use general characteristic ridge patterns produced by fingerprints to easily identified individuals.

There are two different types of fingerprints:

Visible prints make an impression on surfaces like blood, dirt or clay.
We produce latent prints when sweat, oil and other substances on the skin reproduce the ridge structure of the fingerprints on any surface we touch, such as glasses or the murder weapon used by a perpetrator. These prints are usually invisible to the naked eye, and so we use dark powder, lasers, or other light sources to see them. Police officers can "lift" these prints with tape or take special photographs of them.

Footwear & Tyre Track Examination

Investigators split footwear and tyre tracks, which are left behind on most surfaces, into three types:

Visible: a transfer of material from the shoe or tyre to the surface that can be seen by the naked eye without additional aids. For example, bloody shoe prints left on flooring or tracks left by muddy tyres on a driveway. Investigators will lift the entire object on which the print is imprinted upon, if possible, by using a lifting technique, such as:
- Adhesive lifter: a heavy coating of adhesive, usually used in conjunction with fingerprint powders. This method lifts the imprint from smooth, non-delicate surfaces such as tile or hardwood floors.
- Gelatin lifter: a sheet of rubber with a low-adhesive gelatin layer on one side. The gelatin can lift prints from almost any surface, including porous, rough, curved and textured surfaces. It is less tacky and more flexible than an adhesive lifter, allowing it to pick up a dusty shoeprint on a cardboard box, for example, without damaging the box surface.
- Electrostatic dust-print lifting device: this tool electrostatically charges particles within the dust or light soil, which are then attracted and bonded to a lifting film. This method is best for collecting dry or dusty residue impressions on almost any surface, even the skin of a corpse.
Plastic: a three-dimensional impression left on a soft surface. These prints include shoes or tyre tracks left in sand, mud or snow. They can be collected using casting, which involves the use of a powdered stone material mixed with water which is then poured into the impression, creating a three-dimensional model of the impression when it dries.
Latent: not readily visible to the naked eye and created through static charges between the sole or tread and the surface. Examiners or investigators use powders, chemicals or alternate light sources to find these prints. Examples include shoeprints detected on a tile or hardwood floor, window sill, metal counters, or tyre tracks found on road surfaces, driveways or footpaths.

Hair

Examination of hair from a crime scene or suspect can yield a wealth of information. The location of individuals can be traced by using hair to indicate contact with surfaces or individuals. Examination of the root structure of hair shows whether hair has fallen out or forcefully removed, indicating a struggle. Hair analysis can yield information about the ethnicity, sex, the age of its owner, and if the root is present, may also be used to help identify individuals through DNA analysis. The absorbent property of hair makes its examination important in cases of arsenic poisoning. Hair picks up the poisons from the bloodstream, and it is possible to work out the approximate strength and frequency of the dosage by analysis.

Other trace evidence

There are many other types of trace evidence. If you would like to include some of them in the lesson, you can find further information here: https://www.australianpolice.com.au/forensic-scientists/

3. Medical (analysing the evidence)

Medical forensic examiners complete a wide range of tasks, such as:

Forensic biologists look at the genetics of any evidence left at a crime scene and conduct blood typing and DNA profiling. These tests help to identify people at the scene of a crime and to match suspects to evidence. This concept is explored further in the Cracking The Case With The Code lesson.
Forensic chemists analyse any substances (e.g. explosives, drugs) found at the scene of a crime. Where police seize drugs or suspect poisoning, they will call a specialised chemist called a toxicologist to determine the type of drugs based on their poisonous residue.
Forensic odontology/dentistry investigators study the dental evidence found at a crime scene and help identify or age unknown corpses. They can also provide an assessment of bite mark injuries.
Forensic pathologists carry out post mortem medical examinations on dead bodies to determine things such as the cause of death, wounds and injuries.
Forensic anthropologists often work in criminal cases where there are only skeletal remains to help identify bodies that are decomposed, burned or otherwise unrecognisable. By analysing the age of the bones and the marks on them, they can help establish the time of death and the types of wounds sustained by a victim.
Forensic psychiatrists are medically trained doctors who can evaluate a person’s mental state at the time of an offence and/or during their trial, give their opinion about any psychological issues and provide psychological profiles of unknown perpetrators.
Forensic psychologists are not medical doctors but can perform counselling, assist in cases such as child custody disputes and child abuse, advise judges on the state of defendants and victims’ mental health, and provide criminal profiles. This role is explored further in the “Forensic Psychology” lesson.

For further background information on Forensic Science, please visit:

Real-life CSI - DIY Crime Scene

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