Crime scene, as the name suggests, refers to a location at which a criminal act has been committed. Crime scene investigation involves preserving, processing and reconstructing a crime scene.
It is impossible for any person to enter a crime scene without leaving some trace behind, even if that person does not touch anything. This fact is the premise of what is known as Locard’s Principle. Crime scene investigations attempt to eliminate contamination by limiting the number of people allowed to enter a crime scene, and by controlling how they move through it. But it is not possible to totally eliminate contamination, and so crime scene investigators must record and demonstrate ongoing contamination by those who have already moved through the crime scene.
Upon arriving at a crime scene, it is the responsibility of the first responding officer or investigator to establish a pathway of authorized personnel through the crime scene. That is, the first investigator will determine a pathway of how all others will re-enter the crime scene once it has been “locked down”. He or she will photograph the proposed area that will be the path of contamination and, dressed in sterile crime scene attire, will enter the scene to make that determination.
Once the pathway is established, all other police and forensic specialists will move through the crime scene in one of several search patterns. The choice of which pattern to use will depend on what is needed from the scene – a lancet for blood analysis, plaster casts or photographs for instance. All identified evidence is logged in a crime scene log and secured using techniques that protect it from becoming damaged or contaminated.