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Crime scene investigation is an important step in forensic science in which physical evidence is identified, collected and preserved well to ensure that the assailant is identified and prosecuted. Linking of the crime physical evidence with the victim, crime scene and the suspect will ensure effective criminal prosecution. The crime scene is a place where evidential information can be obtained, and it originates with the victims. It can either be primary, secondary, or tertiary depending on the exact point where the crime was committed. For instance, a crime scene is referred to as primary if the victim is found were the crime was originally committed. If the victim is transported by car to another place, the car becomes a secondary crime scene while the place of destination is a tertiary crime scene. When the victim is alive and conscious, it becomes easier for an investigator to determine whether the crime scene is primary, secondary or tertiary (Dale & Becker, 2007).
The sexual assault involved two young girls as the victims. These victims were still on the crime scene but the assailant, who was the step father of the two girls, had already left. The possible physical evidence that were collected from the crime scene included the offender's hair, semen which is due to ejaculation, saliva, and blood stains (Dale & Becker, 2007). Some evidence such as semen, saliva and blood stains are usually found on the victims' clothes as well as the beddings especially the bedcovers. The victims become the source of oral, anal or vaginal swabs that need to be dried well for preservation. It will be very important for the physical evidence to be collected carefully and then preserved well to avoid any kind of alteration. The presence of the victims on the crime scene easily helps the crime scene investigator to directly link the victims to the crime scene. According to Becker (2008), the offender will be the last and very party to be linked to both the victims and the physical evidence collected at the crime scene. The linkage of the offender both to the physical evidence and the victim will complete the evidence linkage triangle which will enhance effective prosecution of the assailant (Becker, 2008).
So that the physical evidence is collected the investigators should be aware that various crime scenes need different ways of evidence collection and this can also be determined by various crime laboratories. A sexual assault evidence kit which is a collection the biological evidential materials from the sexually abused victim, aids in the process of arresting and convicting an offender (Becker, 2008). It is important that the chain of evidence is maintained when the sexual assault evidence kit is open. The contents of a sexual assault kit usually consists of check-off sheet and instructions; the victims' clothing; oral, virginal and anal swabbing; cervical smear; blood samples; physical and history documentation forms; and pubic hair (Eunice, 2009). For purposes of typing, a blood sample must be obtained from the victims. Once the physical evidence is collected from the crime scene and preserved well, it is availed to the crime laboratories for forensic scientists to process the evidence.
When the offender will be taken into police custody, a process known as offender profiling is used to link the offender to the victim and the crime scene so as to complete the linkage triangle. The offender profiling which is also known as criminal profiling is defined as a method used to identify the assailant based on the analysis of the type of the offence and the way in which the offence was committed (Eunice, 2009). A number of aspects of the assailant's personality makeup are ascertained from his decisions before, during, and after the offence. The psychological information is used together with the physical evidence to fully describe the assailant. The physical evidence directly links the victim to the crime scene and the assailant while the psychological information shows why the situation occurred (Becker, 2008). The analysis of physical evidence and psychological information forms the basis to effectively ascertain the offender and prosecute him. The three parties are therefore very important sources of crime evidence and must all be used together for a meaningful and successful process of investigation.