Crime scene investigation (CSI) is the process in which the physical evidence from a crime scene is gathered. Its primary purposes are to identify the victim and possible suspects, carefully collect evidence that suggests how the crime was committed and prepare the evidence for presentation in a court. Experts in different fields are typically employed to make sure the gathered information is truthful and has not been tampered with.
Before the crime scene investigation unit can document a scene, the police typically secure the area and make sure there is no outside interference. If the evidence requires special analysis, a specialist in the field of forensic science may be called to the site. Homicides usually require a medical examiner to determine the cause of death, but sometimes laboratory tests are needed to be certain of it. Detectives are also usually on the scene to start putting the events of the crime together, which commonly includes interviewing witnesses and consulting with the CSI unit.
The investigation process must be conducted with extreme care and close attention to detail. With this in mind, the investigators' first step is usually identifying the boundaries of the entire scene and exploring all possible places for clues. Investigators then walk through the scene carefully, to avoid changing anything since the crime was discovered. The site is then documented, which typically involves making drawings or taking pictures or video. Only during the subsequent collecting of evidence are items from the scene actually touched. When collecting evidence, it is carefully tagged and preserved.
Many different types of clues may be found, depending on the nature of the crime. For example, if a murder has taken place, the exact position of the body is usually outlined and measured. Bodily fluids as well as any cuts, broken bones, bruises, and other physical wounds on a murder victim can be very telling as to how he or she was killed, therefore evidence such as this is carefully noted. The position of the victim's clothing may suggest information about the crime, such as whether the body was moved from one location to another. In such a case, and many others, it is crucial to determine the exact location of the crime.
Other pieces of evidence can include broken glass, fingerprints, weapons, and bullets. Personal belongings like phones, computers, and diaries can also be very helpful in determining what happened. Even if the perpetrator took great effort to hide his or her deeds, it is extremely hard to fully conceal all clues so that a crime scene investigation yields no results.
Preserving and collecting physical evidence is called the mechanical aspect of a CSI, and is fundamental to every investigation. The thinking aspect, on the other hand, involves trying to understand what the evidence means and how it might explain the crime. The detective is heavily involved in this process, though his or her work may stretch on for a long period of time.
The evidence gathered in a crime scene investigation is normally processed at a crime lab by forensic scientists. Although the two fields overlap, a CSI unit is not necessarily the same as a forensic unit. A medical examiner can be considered as part of the forensics team and will typically submit reports of the exact time and cause of death to the detective, to aid in his or her investigation.