A forensic scientist at a crime scene secures, documents, and collects every piece of physical evidence that could link a suspect to a victim or reconstruct what happened. Their work follows a deliberate sequence, from an initial walkthrough to the final sealing of evidence bags, and every step is designed to preserve the integrity of evidence so it holds up in court.
Arrival and Initial Walkthrough
Before touching anything, a forensic scientist gets briefed by the first officers who responded. This conversation covers what’s already known: who called it in, what’s been moved, who has entered the area, and any immediate safety concerns. The scientist then reviews the scene boundaries that officers set up, often adjusting them outward if evidence extends beyond the initial perimeter. The boundary needs to enclose the furthest piece of physical evidence connected to the event.
Next comes the walkthrough. Wearing full protective gear (double-layered latex gloves, shoe covers, a gown, mask, and goggles), the scientist moves carefully through the scene to get an overview without disturbing anything. The goals are specific: identify hazards, spot fragile evidence that could be lost or degraded, note environmental conditions, and start forming a mental map of spatial relationships. This assessment determines what additional specialists or equipment might be needed.
Documenting the Scene
Documentation begins before any evidence is collected, and it never really stops. The forensic scientist takes continuous notes throughout the investigation, recording details that might seem minor but often prove critical later: lighting conditions, whether doors and windows were open or closed, odors in the air, signs of recent activity like food preparation, and date and time indicators such as newspapers or mail.
Photography follows a strict three-tier approach. Overall photographs capture the scene as the scientist first encountered it, including exterior shots of addresses, street signs, and landmarks that establish location. These “as-is” images are taken before anything is added to the scene, including evidence markers. Mid-range photographs show how items relate to each other spatially, framing evidence alongside fixed reference points like furniture or doorways. Close-up photographs fill the entire frame with a single item, maximizing detail. When the size of an object matters, the scientist places a millimeter scale next to it for reference, but always takes a photo without the scale first so nothing obscures the evidence.
Sketches complement the photographs because cameras can distort spatial relationships. At the scene, the scientist draws rough sketches showing measurements and the positions of evidence relative to walls, doors, and other fixed points. These come in several forms: bird’s-eye views showing the full layout from above, elevation sketches showing vertical relationships, and cross-projection sketches that “unfold” walls flat to show evidence placement on both floors and walls. Later, these hand-drawn versions get converted into polished computer-aided design diagrams for courtroom presentation.
Collecting Biological Evidence
Blood, saliva, and other bodily fluids are among the most valuable types of evidence because they carry DNA. The collection method depends on the surface and condition of the stain. For dried blood on a hard surface, the scientist may use a sterile swab slightly moistened with distilled water, pressing and rubbing the stain to concentrate it on the swab tip. Some protocols call for a second dry swab afterward to pick up any remaining material. Alternatively, the scientist can cut out a section of the stained material with a sterile blade, or scrape dried deposits onto clean paper using a razor.
Every biological sample must air-dry before packaging. Sealing a damp sample in plastic creates a warm, humid environment where bacteria break down DNA rapidly. Paper envelopes and bags are standard because they allow airflow. If a body is present, the scientist conducts a superficial external examination without removing clothing, documenting visible injuries, the position of the body relative to its surroundings, and any evidence on the body itself. Blood and fluid samples may be collected from the body at the scene before remains are transported for autopsy.
Recovering Trace Evidence
Trace evidence is anything microscopic or nearly so: fibers, hair, glass fragments, soil, paint chips. Finding it requires careful visual searches, often aided by oblique lighting (shining light at a sharp angle to make tiny particles cast visible shadows) or alternate light sources like ultraviolet lamps that cause certain materials to glow.
Recovery techniques range from precise to broad. Picking with clean forceps is the most targeted approach, used when the scientist can see and isolate a specific fiber or fragment. Tape lifting involves pressing adhesive tape firmly and repeatedly across a surface to pull up loosely attached material, then placing the tape onto a transparent backing for viewing. Scraping with a clean spatula dislodges particles onto collection paper. Combing recovers trace evidence from a person’s hair. Fingernail clippings and scrapings capture material trapped under the nails, with left and right hands packaged separately. Vacuum sweeping, using a vacuum fitted with a filter trap, is typically saved for last because it’s indiscriminate and collects large amounts of irrelevant debris along with potential evidence.
The guiding principle is to use the most direct and least intrusive method practical. Every tool that contacts evidence (forceps, combs, blades) must be clean or sterile, and changed between samples to prevent cross-contamination.
Processing Fingerprints
Latent fingerprints, the invisible ones left by natural oils and sweat on skin, require chemical or physical development to become visible. The method depends on whether the surface is porous or non-porous.
On non-porous surfaces like glass, plastic, or metal, powder dusting is the most common field technique. The scientist applies fine powder with a brush, and the powder sticks to the oily residue of the print. Regular powders use colorants that contrast with the surface, while magnetic powders use a magnetic applicator that reduces brush contact and minimizes smearing. Fluorescent powders glow under alternate light sources, which helps on multicolored or patterned surfaces where standard contrast is poor. Another option for non-porous surfaces is cyanoacrylate fuming, essentially exposing the area to superglue vapor, which polymerizes on fingerprint residue and turns the print white and visible.
On porous surfaces like paper, cardboard, or raw wood, chemical methods target the amino acids naturally present in fingerprint residue. One widely used chemical reacts with these amino acids to produce a visible purple stain. These porous-surface techniques often require controlled conditions and are more commonly applied in the lab, though the scientist at the scene will identify and carefully package items likely to carry prints for later processing.
Bloodstain Pattern Analysis
When blood is present at a scene, its patterns tell a story about what happened and in what order. A forensic scientist trained in bloodstain pattern analysis examines each stain’s shape, size, and directionality through high-resolution photography. Passive stains from gravitational dripping look very different from transfer marks (smears left when a bloody object contacts a surface) or projected patterns caused by forceful impacts.
The scientist also looks for void patterns, unstained gaps within an otherwise continuous blood distribution. These can indicate that an object or person was in that spot during the bloodshed and was later moved. Drip trails reveal the direction and speed of movement. The presence or absence of fine mist-like spatters helps distinguish between different types of force. All of this analysis feeds into reconstructing the sequence of events and can reveal whether a scene has been staged or altered after the fact.
Casting Impressions
Footprints, tire tracks, and tool marks in soft surfaces like soil, mud, or snow are preserved by casting. The scientist mixes dental stone (a type of plaster) and carefully pours it into the impression, sometimes using frames to contain the material. Once it hardens, the cast captures a three-dimensional replica of the impression, including fine details like tread wear patterns or tool striations that can be matched to a specific shoe, tire, or implement.
Maintaining the Chain of Custody
None of the evidence collected matters if the chain of custody breaks. Every item gets a unique identification code and a label recording the location where it was found, the date and time of collection, and the name and signature of the person who collected it along with a witness signature. From the moment evidence is sealed in its container, every transfer between people is logged. This unbroken record proves that the evidence presented in court is the same material recovered at the scene and that it hasn’t been tampered with.
Scene Debriefing
Before leaving, the forensic scientist participates in a debriefing with detectives and other responders. This is where the team shares observations, flags evidence that needs priority processing at the lab, and coordinates next steps. If the case involves a death, the scientist communicates with the pathologist about autopsy scheduling and any findings from the scene that should guide the examination. The scene is then secured, and the collected evidence moves into the lab phase, where analysis transforms physical material into the scientific findings that can ultimately be presented in court.