How Crime Scene Investigation Works

Step 1: Secure the Scene

The first officer to arrive establishes a secure perimeter around the crime scene using barrier tape, vehicles, or officers stationed at access points. The perimeter must be large enough to encompass all potential evidence, which may extend well beyond the immediate area of the crime. In outdoor scenes, investigators typically set the perimeter 50 to 100 feet beyond the visible limits of evidence, then adjust as needed.

A crime scene log records every person who enters or exits the scene, along with the time and purpose of their access. This log becomes a critical document if defense attorneys later challenge evidence integrity. Only essential personnel are allowed inside: investigators, evidence technicians, the medical examiner (for death scenes), and any specialists requested by the lead investigator.

First responders also attend to any victims, taking care to document changes they make to the scene while providing medical assistance. If an officer moves a body to check for vital signs, that movement is noted. If paramedics cut clothing to treat wounds, the cut locations are documented. These records prevent defense challenges based on scene alteration.

Step 2: Conduct a Preliminary Walkthrough

The lead investigator or crime scene supervisor enters the scene along a designated path, carefully observing without touching anything. This walkthrough serves several purposes: assessing the overall condition of the scene, identifying obvious evidence, determining the type and extent of processing needed, and establishing a safe entry and exit route that avoids evidence.

During the walkthrough, the investigator develops a theory of what happened based on the scene layout, visible evidence, and victim information. This is not a final conclusion but a working hypothesis that guides the search strategy. A shooting scene with shell casings requires different processing than a burglary scene with tool marks on a window frame or an arson scene with accelerant residue.

The walkthrough also identifies potential hazards. Crime scenes may contain biological hazards (blood, bodily fluids), chemical hazards (drug manufacturing chemicals, carbon monoxide), structural hazards (fire-damaged buildings), or weapons. Investigators wear appropriate personal protective equipment (PPE) including gloves, shoe covers, and sometimes full Tyvek suits and respiratory protection.

Step 3: Document the Scene

Documentation is the most important phase of crime scene processing because it creates a permanent record of conditions that can never be replicated. Three forms of documentation are standard: photography, videography, and sketching.

Photography follows a progression from general to specific. Wide-angle photographs capture the overall scene and its relationship to surrounding areas. Mid-range photographs show specific evidence items in context with nearby landmarks. Close-up photographs capture detail, always accompanied by a scale (ruler) for size reference. A major crime scene generates hundreds to thousands of photographs. Each photograph is logged with the photographer's name, date, time, camera settings, and a description of what the image depicts.

Crime scene sketches provide accurate spatial relationships that photographs alone cannot convey. Measurements are taken from fixed reference points (walls, permanent structures) to each evidence item using baseline or triangulation methods. Modern crime scenes increasingly use 3D laser scanning (LIDAR) that captures millions of data points, creating a photorealistic digital model accurate to within 2 millimeters. The digital model allows investigators to revisit the scene virtually, take new measurements, and create courtroom presentations long after the physical scene has been released.

Videography records a continuous walkthrough of the scene, providing context that still photographs may miss. The videographer narrates the recording, describing what is visible and moving through the scene at a pace that allows viewers to understand spatial relationships.

Step 4: Search for and Collect Evidence

Systematic search patterns ensure nothing is overlooked. In the grid method, investigators divide the area into sectors and search each one methodically. In the spiral method, they move outward from the center or inward from the perimeter. In the strip or lane method, they walk parallel paths across the scene. The choice depends on the scene size, environment, and number of available investigators.

Each piece of evidence is photographed in place, assigned a unique evidence number, and its location marked on the scene sketch before collection. The investigator documents the condition of the evidence, noting details like whether a door was open or closed, whether lights were on or off, and whether items appear disturbed.

Biological evidence requires special handling. Blood samples are collected using sterile swabs moistened with distilled water. Wet biological evidence must be air-dried before packaging to prevent bacterial degradation of DNA. All biological evidence goes into paper packaging (bags or envelopes), never plastic, which traps moisture and promotes mold growth that destroys DNA. The only exception is evidence already in liquid form, which goes into leak-proof containers.

Firearms are made safe (unloaded) with the condition documented, including the position of the safety, the number of rounds in the magazine, and whether a round was chambered. The barrel is protected to preserve any biological evidence or bullet fragments. Cartridge cases and projectiles are collected individually and packaged to prevent contact that could alter their surface markings.

Step 5: Process Latent Evidence

Latent fingerprints, those invisible to the naked eye, require development before collection. The technique depends on the surface type. Smooth, nonporous surfaces (glass, polished metal, plastic) are dusted with fine powder that adheres to the oils and amino acids in fingerprint residue. The developed print is photographed, then lifted using adhesive tape and placed on a contrasting card.

Porous surfaces (paper, cardboard, raw wood) require chemical processing. Ninhydrin reacts with amino acids in fingerprint residue to produce a purple color. DFO (1,8-diazafluoren-9-one) produces fluorescent prints visible under alternate light sources. These chemicals are applied by dipping, spraying, or fuming, and the results may take hours to days to develop fully.

Cyanoacrylate (superglue) fuming is one of the most versatile techniques. In a sealed chamber, heated cyanoacrylate releases fumes that polymerize on fingerprint ridges, forming a hard white deposit. This technique works on a wide range of surfaces and stabilizes the print for subsequent powder or dye staining that enhances contrast. It is standard processing for items submitted to the laboratory.

Alternate light sources (ALS) reveal evidence invisible under normal lighting. Body fluids, certain fibers, bruising patterns, and trace chemicals fluoresce or absorb at specific wavelengths. Investigators scan surfaces with the ALS at multiple wavelengths while wearing barrier filter goggles, marking any responsive areas for collection.

Step 6: Package and Submit Evidence

Proper packaging prevents contamination, degradation, and cross-transfer between evidence items. Each item is packaged separately. Sharp objects like knives go into rigid containers like boxes or tubes, with the blade secured to prevent movement. Clothing from different individuals is packaged in separate paper bags. Soil samples go into clean containers. Digital devices are packaged in Faraday bags that block wireless signals, preventing remote wiping or data alteration.

Chain of custody documentation accompanies each evidence item from collection through laboratory analysis, storage, courtroom presentation, and eventual disposition. The chain records every person who handled the evidence, the date and time of each transfer, and the purpose. Any break in the chain creates a legal vulnerability that defense attorneys will exploit.

Evidence submission to the laboratory includes a formal request specifying what examinations are needed. A blood-stained shirt might be submitted for DNA analysis, bloodstain pattern interpretation, and fiber comparison. The laboratory prioritizes cases based on factors including the seriousness of the offense, whether a suspect is in custody, and court deadlines.