Crime Scene Reconstruction (World of Forensic Science)
The process of working out the sequence of events before, during, and after a crime is known as crime scene reconstruction. It is perhaps one of the aspects of forensic science that fascinates the public most, featuring in most police dramas. Reconstruction requires not just a scientific approach but also logic, experience, and open-mindedness on the part of the investigating team who must be prepared to set aside any hypothesis that does not fit with the actual evidence presented to them.
Reconstruction starts when the investigator takes a first walk through the scene where the crime took place. Even at this stage, it may be possible to construct a rough hypothesis of what may have happened and how. A hypothesis is a set of ideas or a general picture of what may have happened. It does not become a theory until it fits all the available evidence and supporting information.
While the investigator is forming a first impression, others are recording the scene and gathering evidence. Crime scenes vary enormously, from a petty theft or break-in to violent crime that may involve fire or explosions. The principles of investigation remain the same, although the investment of time and energy into it will vary with the seriousness of the crime. The investigator will want to establish who was involvedhat is, what are the identities of the victim, perpetrator, and witnesses. They also need to know where, when, how, and why the crime took place.
The crime scene is first documented through note-taking, video, photography, and sketching. The investigating team will then search for, record, collect, and take away various kinds of evidence such as toolmarks, hair, bloodstains, fibers, and footprints. According to Locard's exchange principle, every contact leaves a trace. That is, those involved in the crime always leave something behind or take something with them. Think of putting your hand on a patch of wet paint. The hand-print may be clearly visible. You will also have paint stains on your hand. Evidence of this kind in a crime situation is known as trace evidence and consists of tiny amounts of substances like fibers, paint, mud, soil, or blood. Often is it only visible through a microscope and needs specialist laboratory investigation to assess its significance to the investigation.
To render trace and other types of evidence valid and admissible to the court, it is essential to have strict control of how the site is investigated to avoid undue interference or contamination. That is why access to the crime scene has to be limited and those involved will always proceed from the police cordon to the site of the crime itself down a common approach path, which will be set so as to allow minimal interference with any evidence.
Everyone who handles a piece of evidence is recorded and hands it on the next in line so that a tight chain of custodyrom the scene to the laboratory and, eventually, the court rooms created. There are special ways of transporting evidence to protect it. Dry trace evidence, such as hairs and fibers, might be placed in druggists' folds, which are small, folded papers. Wet evidence, including bloody clothing, has to be allowed to air dry because moisture can attract molds that might decay the specimen, rendering it useless. After being placed inside an appropriate primary container, pieces of evidence are then placed inside a larger container, completely sealed with tamper-proof tape and carefully labeled. Each item is packed separately to prevent cross-contamination, which could otherwise destroy the credibility of the evidence. In the case of toolmarks on points of entry, it may even be necessary to remove a whole door or window rather than attempt to excise the mark, which may damage it. Once in the forensic laboratory all the pieces of evidence are then analyzed and interpreted.
Bloodstain patterns are a vital aid to the reconstruction of a violent crime. When blood drips from wounds, weapons, or other objects, a splash or spatter pattern is created. The shape of the splash can show whether the source of the blood was moving and, if so, in which direction. Should the victim or perpetrator attempt to run away, the trail of blood will tell the investigator more details about the escape attempt because the shape of the blood drops will help reveal it. When someone is shot or hit with a blunt object, blood is projected from the wound and hits surrounding surfaces and objects. The resulting pattern can be analyzed to show how the weapon impacted the victim. There may be a break in the pattern; no blood where it would be expected on, for instance, a wall. This may suggest where the attacker was standing and whether the victim was struck or shot from the front or behind.
Blood is messy. During an attack it is also transferred to clothes, shoes, and hands and may leave behind bloody prints. Blood-soaked fabric makes marks with a characteristic weave pattern upon objects it comes into contact with, like a getaway car.
Some forensic tools are particularly important in crime scene reconstruction. Fluorescent light sources will glow when they are exposed to ultraviolet light. An important example is luminol, a chemical that reacts with hemoglobin, the red pigment in blood. Luminol detects blood at a concentration as low as one part in five to ten million or even lower. It is extremely valuable in revealing blood that the perpetrator believes he or she has cleaned away, such as bloodstains in a car used to remove the victim's body. However, luminol cannot detect bloodstains that have been wiped away with bleach. In such cases, fluorescein can be used as an alternative. In other words, fluorescent chemicals and light can be used to detect invisible trace evidence, giving a truer picture of the crime scene.
Footprints are a particularly rich source of evidence in a reconstruction because they can link a suspect to the crime scene. Even if a suspect says they were not there, their footprints, if matched, can tell a different story. Footprints have proved to be especially important in cases of homicide, assault, robbery, or rape. When someone is at the scene of the crime, their soles come into contact with surfaces and leave an imprint, visible or not, which can be detected, examined, and assessed. Some prints, such as those made in the blood of a victim, are particularly obvious. If the print is in contact with a soft surface such as sand, soil, or snow, it will leave a three dimensional print. Should the contact be with a hard surface, the print is two-dimensional. Either the surface itself is removed and taken to the laboratory, or specialized photographs are made. The footprint can be linked to a particular kind of shoe by comparison with a footwear database. Individuals also wear down their shoes in a certain way, depending upon their gait. Other features such as scuffmarks can also be identified within the prints. Unless a suspect has had the foresight to destroy their footwear, examination of their shoes and comparison with footprints found at the scene can link them with the crime.
The investigators often carry out their own experiments to test the hypothesis. For instance, in establishing the relative location of victim and perpetrator in a shooting incident, it is important to know the distance between the gun and the point of impact. Was the victim shot from in front or behind? Was the suspect shooting at point blank range or from a distance? Simulation experiments to solve this question would involve shooting an identical weapon from different distances at a laboratory target. The resulting damage from the bullet could then be compared to that found at the actual scene of the crime.
The investigators must then relate all the evidence they have collected and analyzed with other information, such as autopsy reports and witness statements, continually refining or even rejecting their original hypothesis. The autopsy may show, for example, the time of death and whether the body has been moved. A witness statement may not be consistent with the evidence, which may provide a basis for further interrogation with questions directed by the interpretation of the evidence. This process will generate new information to be fitted into the hypothesis.
New information may continue to come in and must be examined to see if it is consistent with the hypothesis. A murder weapon or even a body may be found during the investigation. Maybe a witness will change or add to their statement. The final reconstruction is the investigator's presentation of the sequence of events before, during, and after the crime. It gives the location and position of everyone involved. More important, it tells how and why the crime occurred. The investigators can expect to be challenged in court, of course. While investigators
SEE ALSO Crime scene investigation; Crime scene staging; Evidence, chain of custody.