Back in the day, law enforcers thought that the most important evidence in a crime was that provided by eye witnesses. However, eye witnesses are in fact part of the evidence, and what they have witnessed may have been so distressing as to affect their evidence. In addition, evidence given by a person has an element of subjectivity due to varying observation skills, and any preconceptions and prejudices the witness may entertain. In order to solve a crime, the investigation team require evidence that tells it as it is, and this is where forensics come in.
Forensic evidence is the most important element in bringing a perpetrator of a crime to justice. As technology has advanced, many of the types of evidence the forensic expert collects can be attributed to a single person, with no likelihood that the evidence could be applied to anyone else. Fingerprints and DNA samples left at the scene are unique, and are often the means of securing a conviction, and the forensic expert has a number of skills he can call on to collect evidence from other sources.
The popularity of television crime shows such as CSI and Silent Witness has demystified the science of forensics to the extent that half the viewing public see themselves as an armchair forensics expert. However, this is how the real forensic experts go about collecting incriminating evidence.
Securing the crime scene
Every crime has one or more crime scenes, and before the forensic expert can collect evidence, the crime scene must be secured. This involves sectioning off the area, keeping members of the public out, and ensuring that anyone who enters the crime scene is wearing protective clothing. This practice ensure that hairs and fibres from the investigation team don't contaminate the crime scene, and avoids introducing materials from outside the crime scene that could mislead the forensic team.
Photographing the crime scene
Before anything can be removed from the crime scene - including evidence and bodies - the crime scene must be photographed in detail. This helps to give investigating officers who were not present at the crime scene a visual representation, and is also useful in the event that witness statements conflict or contradict each other.
Evidence is numbered with special indicators which also indicate heights and distances. This allows investigating officers to cross-reference evidence with reports and serves to identify the evidence as exhibits in court when the case goes to trial. These days, a video is often used to record the crime scene. Forensic photography gives the police an overview of the crime scene, and also helps with later reconstructions of the crime or events leading to it.
After photography. important evidence is bagged and sealed in airtight plastic bags to avoid contamination. Each person who comes into contact with the evidence in any way must fill in a 'custody chain' document. This ensures that evidence is not lost or cross-contaminated.
Evidence which is bagged could include samples of bloodstains, which may help to identify the victim and/ or the perpetrator of the crime, and items such as cigarette butts and hair samples, which could provide DNA. Fibre samples may be useful in identifying clothing from the victim or suspect, while soil samples help to track the movements of the victim or suspect before arrival at the crime scene.
Larger items of evidence such as tyre tracks and footprint will be photographed. Then, if they are clear enough, a plaster cast will be made. Tyre casts can help identify the make, age and model of a car, as well as disclosing where it has been through samples of materials which may be lodged in the treads.
A cast of a footprint can yield details such as weight, height and even sex if the print is clear. Before a cast is made, a frame is used to isolate the evidence from other items, then plaster of Paris or dental cement - depending on the depth of the print - is used to make a copy of the evidence.
Forensic experts are able to retrieve evidence from computers and laptops using specialised software. It's also possible to restore a computer's hard drive to its original condition and identify who has owned the computer and what they have viewed. Specialist IT forensic experts can also identify false accounting.
Collecting forensic evidence from computers is a long drawn-out process, but there is a high degree of accuracy attached to the findings, and, combined with other evidence, it can help to build a strong case for the prosecution.
Forensic experts have a number of methods at their disposal for collecting evidence. Because there is a strict protocol for collecting evidence, the dangers of contamination are minimised. In addition, the procedure for evidence paperwork ensures that all evidence - however small - can be cross-referenced with witness statements and reports to build a picture of the crime and how it was committed, as well as providing leads to the perpetrator.
Unlike witness evidence, which can be subjective and flawed, forensic evidence is collected and analysed using scientific procedures and principles, so it is accurate and reliable enough to stand up in a court of law.