Crime scene investigation relies on a structured, methodical approach to ensure that physical evidence is gathered without being contaminated or lost. The “7 S’s” represent a standardized, sequential methodology used in forensic science to guide this process from the moment of arrival until the evidence is ready for the laboratory. This framework establishes a consistent protocol that all responding personnel must follow, providing a systematic way to process any location where a crime is believed to have occurred. Adhering to these steps preserves the integrity of the scene and the physical evidence, which is fundamental to maintaining the admissibility of findings in a court of law.
Initial Actions Upon Arrival
The first officer to reach the scene must Secure the Scene, prioritizing the well-being of all individuals present. Injured persons must receive immediate medical attention, though this action must be carefully managed to prevent the destruction or contamination of evidence. Once safety is confirmed, the first responder isolates the area, typically using police tape, to prevent unauthorized personnel from entering. A detailed security log is immediately started to record the names of every person who enters or leaves the perimeter, helping to account for potential evidence transfer based on Locard’s Exchange Principle.
Following the securing of the location, investigators must Separate the Witnesses present at the scene. This separation prevents witnesses from discussing the events, which could inadvertently lead to memory collusion or a shared, inaccurate narrative. Separating individuals ensures that each testimony remains an independent recollection, preserving the integrity of their personal accounts. Investigators collect initial, unfiltered information by asking each witness questions about the sequence of events, such as when the crime occurred and where they were positioned.
Detailed Documentation and Recording
Once the preparatory steps are complete, the investigative team begins documenting the scene, starting with a preliminary walkthrough known as Scan the Scene. This initial survey allows the lead investigator to form a plan and identify the overall scope. They determine if the location is a primary scene (where the crime occurred) or a secondary scene (related but not the original location). During this scan, investigators may mark obvious evidence locations and prioritize fragile or transient evidence that may degrade quickly.
The next step, See the Scene, involves comprehensive photography to create a permanent visual record before anything is moved or altered. A professional forensic photographer systematically captures the location. They begin with overall, wide-angle images to show the general layout and relation of fixed objects. Mid-range photographs then focus on the spatial relationship between individual items of evidence and fixed points. Finally, detailed close-up images are taken of each piece of evidence, both with and without a measurement scale, to accurately represent its size and condition.
Following the visual capture, investigators Sketch the Scene to provide a measurable, spatial record that photographs cannot fully convey. A rough sketch is created at the location, typically drawn to scale on graph paper, including the orientation using a compass to denote magnetic North. This sketch illustrates the precise position of evidence and fixed objects, which are located using measurement techniques like triangulation or the baseline method. Triangulation involves measuring the distance of an item from two separate, fixed points, such as room corners, to establish its exact location.
The rough sketch is later converted into a final, polished version, often using computer-aided design (CAD) software, which maintains the accuracy of the measurements. This measurable document is a necessary component for the reconstruction of events and for presenting the findings in a legal setting. By providing scaled diagrams alongside the photographs, the documentation phase ensures that the scene’s layout and the context of the evidence are preserved indefinitely before collection activity begins.
Collecting and Preserving Physical Evidence
With documentation complete, the investigation moves to Search for Evidence, requiring the team to employ a systematic pattern to ensure no item is overlooked. The chosen pattern depends on the size and characteristics of the area and the number of personnel available. Patterns include:
- Grid search
- Spiral search
- Line search
- Zone search
The careful selection and execution of a search pattern allows investigators to locate trace evidence, such as hairs, fibers, or latent fingerprints, that may not have been visible during the initial scan.
Once an item is located, the final step is to Secure the Evidence, a procedure focused on maintaining the integrity of the collected material for forensic analysis. Each item must be packaged separately in an appropriate container to prevent cross-contamination. For example, biological materials require breathable paper packaging, while arson evidence needs airtight containers to preserve volatile residues. Fragile items like dried blood or trace materials are often collected in a “druggist’s fold,” a specialized paper packet, before being placed into a larger evidence bag.
The packaging is then sealed with tamper-evident tape, and a detailed label is affixed, recording the case number, date and time of collection, a description of the item, and the collector’s signature. This initial labeling establishes the Chain of Custody, a chronological paper trail that documents every person who subsequently handles the evidence. Maintaining an unbroken chain of custody is a procedural requirement that legally verifies the authenticity of the evidence, ensuring it has not been altered or substituted before its presentation in court.