Evidence is the foundation of every forensic investigation. How it is collected, documented, and preserved determines whether it is admissible in court.
Crime Scene Processing Workflow
Secure Scene
Document
Photo / Video / Sketch
Collect Evidence
Package & Label
Transport to Lab
Chain of Custody
Evidence found at crime scenes falls into four broad categories, each requiring specialised collection and analysis techniques.
Tangible items that can be observed and measured — fingerprints, fibers, tool marks, glass fragments, and shoe prints. Physical evidence can link a suspect to a scene or establish how a crime was committed.
Evidence of biological origin — blood, saliva, semen, hair, and skin cells. Biological evidence is particularly valuable because it can yield DNA profiles that identify individuals with near-absolute certainty.
Substances identified through chemical analysis — illicit drugs, fire accelerants, gunshot residue (GSR), and poisons. Detection often relies on techniques such as mass spectrometry and chromatography.
Data recovered from electronic devices — phone records, CCTV footage, computer files, GPS data, and social media activity. Digital forensics has become increasingly critical as technology pervades modern life.
Quick Check
Which type of evidence would gunshot residue (GSR) be classified as?
The chain of custody is the documented trail that records every person who has handled a piece of evidence from the moment it is collected at the crime scene to its presentation in court. It is one of the most critical aspects of forensic investigation.
Without a properly maintained chain of custody, evidence may be deemed inadmissible — regardless of how compelling it might be. Defence attorneys routinely challenge evidence by arguing that gaps in the chain of custody could have allowed tampering, contamination, or substitution.
Gaps in documentation — periods where no record exists of who held the evidence
Improper storage — evidence stored in conditions that could cause degradation or contamination
Unsigned transfers — evidence changing hands without a logged date, time, and handler's signature
Every transfer of evidence must be logged with the date, time, purpose of the transfer, and the signature of both the person releasing and the person receiving the item.
Fill in the Blank
The documented trail that records every person who handled a piece of evidence is called the________.
Before any evidence is collected, the crime scene must be thoroughly documented to create a permanent record of the scene as it was found. Three primary methods are used.
Photographs are taken at three levels — overall shots to capture the entire scene and its surroundings, mid-range shots to show the spatial relationship between items of evidence, and close-up shots with scale references (rulers or markers) to document individual items in detail.
Video recordings provide a continuous walkthrough of the scene, capturing spatial relationships and the overall layout in a way that individual photographs cannot. They complement still photography and often include narration by the lead investigator.
Crime scene sketches include precise measurements and spatial relationships between evidence, landmarks, and entry/exit points. A rough sketch is drawn at the scene with measurements; a finished sketch is later prepared in the lab to scale for use in court.
“The crime scene tells a story. The investigator's job is to read that story before it is lost. Once evidence is contaminated, moved, or destroyed, the story can never be fully reconstructed.”
— Henry Lee, Forensic Scientist
Different types of evidence require specialised collection methods to ensure they are preserved intact and free from contamination.
Sterile swabs are used to collect blood, saliva, and other body fluids. Swabs are air-dried to prevent bacterial degradation and packaged in paper (not plastic) to allow moisture to escape.
Latent fingerprints are developed using powders, chemical reagents (ninhydrin, cyanoacrylate fuming), or alternate light sources, then lifted with adhesive tape and transferred to a backing card.
Shoe prints and tire marks are documented photographically with scale markers, then cast using dental stone to create three-dimensional replicas for comparison.
Fibers, hair, glass fragments, and paint chips are collected with tweezers or adhesive lifters. Items are packaged individually in paper folds or small containers to prevent loss or cross-contamination.
Investigators must wear disposable gloves, shoe covers, and masks at every crime scene. Gloves must be changed between handling different items of evidence to prevent cross-contamination — transferring trace material from one exhibit to another. PPE also protects investigators from biohazards such as bloodborne pathogens.
Quick Check
Why must investigators wear gloves and change them frequently at a crime scene?
Fill in the Blank
The three main methods of crime scene documentation are photography, videography, and________.
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