Evidence presented in court is only relevant if it can be shown to have been involved in a crime and to have survived intact until considered by the judge and jury. For example, if a gun, said to be a murder weapon, bears the fingerprints of an investigating police officer, then its value to the investigation is lost. Disturbed evidence refers to any item of evidence that has been interfered with in some way, whether intentionally or unintentionally. It is up to the crime scene investigators and the forensic analysts to make sure that none of their actions lead to a disturbance of evidence.
Before the investigators even arrive at the scene, however, there is already plenty of opportunity for perpetrators and witnesses to interfere with or dispose of evidence. They may carry out some form of crime scene staging to try to disguise what has really happened.
A common form of staging is trying to make a murder look like an accident or suicide. Perhaps a gun will be carefully placed in the victim's hand or beside the body. However, generally the pathologist will know that the nature of the wounds is not consistent with being self-inflicted. A murderer sometimes moves a body into the bathroom and pretends the victim fell after getting out of a bath or shower. Another type of staging is trying to blame a non-existent intruder for an attack. A man who murders his wife, for instance, may break a window, turn over jewelry boxes and drawers, and remove valuable items. Staged burglaries may also be set up as part of an insurance fraud; the owner removes an item and pretends it has been stolen.
Blood is, of course, one of the most significant forms of evidence. The pattern and nature of a bloodstain is not only revealing of the nature of the attack, it may also identify the perpetrator if there is blood on his or her clothing or if there has been a struggle in which his or her blood is left behind. Attackers may be surprised and panicked by the amount of blood some attacks can generate. They may assume that washing it away with a cleanser such as bleach will remove this most telling form of evidence. In fact, it is hard to remove all traces of blood, especially if the perpetrator is in a hurry to get away, and invisible traces nearly always remain. These can be visualized using a luminol spray, which fluoresces in ultra-violet light when it is sprayed on blood.
The first responder, that is, the first officer on the scene of a crime, may well find someone has tried to hide or destroy evidence. It is the officer's job to stop this from happening. Once the investigation team arrives, it becomes a case of ensuring that none of its actions destroy or compromise evidence. Establishing a common approach path or an agreed way of accessing and leaving the scene minimizes traffic of personnel at the scene. A policed cordon around the scene ensures that only those who really need to be there actually enter the scene, reducing the possibility of contaminating or destroying evidence.
Some items of evidence are, by their very nature, fragile, and the investigators must capture them as soon as possible by photography . Examples include shoeprints in snow or tire marks that could be washed away by rain. Sometimes evidence is destroyed by insects or predators. Insects will feed upon a body or lay their eggs in body tissues within hours of death. While their actions may provide valuable information on time of death , they may also render the body less amenable to pathological investigation. When decomposition is advanced, predators may scatter remains and bones, making it harder for a forensic pathologist or anthropologist to work out the identity of the deceased.
Once the investigators are on the scene, they must bear in mind that they are an intrusion and do their best not to degrade or destroy evidence. According to Locard's exchange principle , everyone involved in a crime scene takes something from the scene, such as mud or blood, away with them, and leaves something of himself behind, like a fingerprint . This is as true of the investigators as it is of the perpetrator and witnesses. If they touch areas that are likely to bear fingerprints of suspects, they could contaminate them with their own fingerprints. Accordingly, investigators will wear gloves, masks, protective bodysuits, and overshoes. Evidence should be collected, if possible, on one occasion, which means it is better to collect too much evidence than too little. Returning to the crime scene to collect further evidence sometimes raises questions in court as to whether it was placed there since the crime occurred.
The possibility of cross-contamination is always a big issue when it comes to evaluating evidence. Trace evidence , like hair, fibers , paint, and blood, is by its very nature readily transferred from one item to another. If cross contamination occurs, the source of trace evidence found on a significant item is uncertain. It may have landed there during the crime itself, in which case it is significant and meaningful evidence. However, it is also possible that the evidence was transferred to the item via a third party during the investigation. The only way of avoiding cross contamination is to follow strictly controlled procedures during an investigation.
For instance, suppose hairs or fibers from a victim are found on the clothing of a suspect. Such trace evidence could be highly incriminating. However, an officer may come to the suspect's home from the scene of the crime after having had contact with the victim. Here the officer may pack up the suspect's clothes for examination. Should the suspect come to court, the defense will say that this officer picked up the hair or fibers from the scene of the crime and acted as an intermediary, transferring it to the clothing of the suspect. The investigating team must have procedures in place that ensure this does not happen. Ideally, officers would not attend more than one scene linked to the crime—in this case, the scene itself or the suspect's home—to avoid cross-contamination of this kind. Given there is a finite number of personnel available for each crime investigation, sometimes the same officer will be involved at more than one scene. In such cases, he or she must follow a strict decontamination process between attending different scenes. The cross-contamination charge can always be answered if careful records are kept of the movements of investigators between different scenes.
Once evidence has been collected, it must be handled, packaged, and transferred with scrupulous care to ensure it is not disturbed from its original state. Each piece of evidence is packaged separately in an unused container and then sealed into an outer container and labeled. Obvious sources of cross-contamination should always be kept well separated. For instance, the clothing of the suspect would never been packaged or handled in the same room as that of the victim and the investigators should be able to prove this. Only by following such procedures to prevent cross-contamination can the true value of trace evidence be revealed in court.
Perhaps the most powerful defense against evidence having been disturbed is the chain of custody protocol. Evidence found at the scene of a crime must eventually be presented and questioned in the courtroom. For the evidence to be of use in a trial, it must make the journey from crime scene to court in a validated and secure manner, so that all involved can be assured that it has not been contaminated and that it really is relevant to the crime investigation. That is why crime investigators must follow a routine commonly known as the chain of custody when it comes to collecting and handling evidence.
The first officer to collect an item of evidence, be it a fiber or a bullet, will sign and date either the item itself or its packaging. Clearly, marking an item ensures there is no ambiguity, as packaging could be separated from the evidence itself. However, some types of evidence, such as bullets, may be altered if someone marks them. Bloodstains and fingerprints, among the most valuable items of evidence, often cannot be collected directly, so it is a case of officers signing photographs or transfers and recording what has been done.
Evidence usually goes from the crime scene to the forensic laboratory for examination. The chain of custody does not end here, however. The receiving officer will sign for and date the evidence packages. Everyone who handles the evidence at any stage in the laboratory does likewise until the analysis is complete. After all, there are as many opportunities for compromising evidence in the laboratory as at the crime scene. Evidence may be stored at the wrong temperature or a label could go missing.
The analysis itself may disturb the evidence. Some experiments actually destroy evidence, so care must be taken that all other necessary tests, such as microscopy, have been done before, for instance, pyrolysis, which heats up a paint sample. Ideally, only a tiny sample would be taken for analysis, but if the evidence is only available in minute amounts, sampling may destroy it. Once laboratory investigations have been completed, the evidence will be handed over to the police for storage until its presentation in court.
The receiving police officer will sign for the evidence and it should be stored in a secure area to minimize the risk of interference or loss. The prosecuting lawyer is responsible for the evidence once the case arrives in court. He or she will sign to that effect, and is the last link in the chain of custody. The investigators should have been aware of any disturbance to the evidence before they arrived at the scene. Once they began work, they would have taken every care to protect fragile evidence and minimize any chance of cross-contamination. The chain of custody involves everyone with an interest in investigating the crime. If it has been followed correctly, then the case can proceed with all involved being aware of the precise journey the evidence took from crime scene to the court. This allows the court to judge the case on the basis of the evidence, assured that it has not been disturbed.
see also Bindle paper; Control samples; Crime scene investigation; Cross contamination; Evidence; Evidence, chain of custody; Fibers; Quality control of forensic evidence; Trace evidence.