Unit 2: Physical evidences

Physical evidence is any tangible object, material, or trace that connects a person, place, or thing to a criminal act and can be perceived, measured, or analysed scientifically. Its authority rests on Locard's Exchange Principle (Edmond Locard, c. 1910): "every contact leaves a trace" — when two objects meet, material transfers between them in both directions. Physical evidence is therefore mute, objective, and independent of memory or bias, unlike testimonial evidence.

• Objectivity: does not lie, forget, or grow nervous under cross-examination; its value depends only on correct recognition, collection, and interpretation.
• Transfer basis: governed by Locard's Principle — a hair, fibre, or paint fleck moves from suspect to scene or victim and back.
• Corroborative function: rarely proves guilt alone; it confirms, refutes, or refines a hypothesis and links elements of the crime triangle (suspect–victim–scene).
• Perishability: many traces (blood, prints, biological fluids) degrade with time, heat, and mishandling, making prompt correct handling essential.
• Analysability: carries class or individual information recoverable through laboratory examination.

II. Types and Classification of Physical Evidence

The vocabulary that organises every item recovered at a scene.

A. Types of physical evidences

Physical evidence is grouped by its physical nature and the information it carries.

• Trace evidence: microscopic or minute material — hair, fibres, glass fragments, soil, gunshot residue.
• Biological evidence: blood, semen, saliva, tissue, bone; source of DNA profiling.
• Impression/pattern evidence: fingerprints, footwear marks, tyre marks, tool marks, bite marks.
• Firearm evidence: cartridge cases, bullets, striations, residue.
• Documentary/questioned documents: handwriting, ink, printed matter.
• Chemical and physical objects: drugs, accelerants, weapons, fractured items.

B. Classification

Evidence is classified chiefly by the specificity of the information it yields.

• Class characteristics: properties common to a group, narrowing but not isolating a source.
  • Example: blood group O, a fibre dyed a mass-produced shade, shoe size 9.
• Individual characteristics: features unique to a single source with high certainty.
  • Example: a fingerprint's ridge minutiae, striations on a fired bullet, a fracture match between two glass pieces.
• Other axes: origin (biological vs. non-biological), state (solid, liquid, gaseous), and whether it is transient (odour, temperature), conditional (position of a body), or pattern (spatter).

III. Evidential Function and Weight

Why physical evidence matters from the first response to the verdict.

A. Role of physical evidences in criminal investigation

It steers the enquiry by generating and testing investigative leads.

• Establishing corpus delicti: proves a crime occurred (e.g., forced-entry tool marks, a spent cartridge).
• Linking or excluding: ties a suspect to scene/victim, or eliminates the innocent, reducing wrongful arrest.
• Reconstruction: blood-spatter geometry, glass-fracture direction, and bullet trajectory rebuild the sequence of events.
• Corroboration of witnesses: confirms or contradicts statements and confessions.
• Lead generation: database hits (fingerprints, DNA, firearm marks) point to unknown offenders.

B. Role of physical evidences in trials

In court, physical evidence supplies objective proof that the finder of fact can weigh.

• Admissibility: must be relevant, authenticated, and shown to be the same item collected — hence the chain of custody.
• Expert testimony: forensic examiners interpret findings; the court assesses method reliability and error rate.
• Standard of proof: helps meet "beyond reasonable doubt" by converging with other evidence.
• Resistance to challenge: its objectivity limits defence attacks to handling, contamination, or interpretation rather than credibility.

C. Probative value of physical evidences

Probative value is the degree to which evidence tends to prove or disprove a fact in issue.

• Individualising evidence: highest value — a matching fingerprint or DNA profile can point to one person.
• Class evidence: moderate value — significance rises as more independent class matches accumulate (product rule of associations).
• Factors raising value: rarity of the trait, quality of the sample, sound analytical method, unbroken custody.
• Factors lowering value: contamination, degradation, common (ubiquitous) traits, gaps in documentation.

IV. Crime-Scene Methodology

The disciplined sequence from search to secure laboratory delivery.

A. Searching techniques at crime scene

A systematic search ensures no evidence is overlooked or disturbed; the pattern is chosen to fit the scene's size and shape.

• Spiral (inward/outward): searcher moves in a decreasing or increasing circle; suited to small, open outdoor scenes.
• Strip/line: searchers walk parallel lanes across the area; efficient for large outdoor grounds.
• Grid: two overlapping strip searches at right angles; the double coverage improves thoroughness.
• Zone/quadrant: area divided into sectors, each searched separately; ideal for indoor rooms.
• Wheel/ray: searchers move outward along spokes from a central point; used for small, circular scenes.
• Rule: search proceeds from general to specific and from least to most destructive; the scene is documented (notes, photographs, sketches) before anything is moved.

B. Processing and examination of physical evidences

Processing converts raw items into court-ready, laboratory-analysable exhibits without altering them.

• Recognition and documentation: photograph in place with a scale, sketch, and note position before lifting.
• Collection: use clean, appropriate tools; collect each item separately to prevent cross-transfer.
• Packaging:
  • Dry trace/biological: breathable paper bags or druggist folds — plastic traps moisture and promotes mould.
  • Liquids/volatiles: airtight, unbreakable containers.
• Labelling and sealing: tamper-evident seal signed and dated across the closure; label with case number, item, location, collector.
• Laboratory examination:
  • Physical: microscopy, refractive index, density, morphology.
  • Chemical/instrumental: spectroscopy, chromatography, DNA analysis.
• Comparison: questioned sample matched against a known/reference (control) sample.

C. Maintaining chain of custody

Chain of custody is the documented, unbroken record of who held the evidence, when, and why, from collection to court.

• Purpose: guarantees integrity and identity — proves the exhibit examined is the exhibit seized, unaltered.
• Record contents: date/time, location, collector's identity, description, and every transfer of possession.
• Continuity: each custodian signs on receipt and release; unexplained gaps allow the defence to challenge admissibility.
• Secure storage: sealed, access-controlled evidence room or freezer for perishables.
• Consequence of a break: evidence may be excluded regardless of its analytical significance.

V. Trace and Pattern Evidence Types

Four common evidence classes, each with distinct recovery and comparison methods.

A. Hair

Hair transfers readily and persists, offering class evidence on origin and treatment.

• Structure: three layers — cuticle (outer scales), cortex (pigment-bearing bulk), medulla (central core).
• Comparison points: colour, medullary index (medulla width ÷ shaft width), scale pattern, human vs. animal origin.
• Root state: an anagen (growing) root with follicular tissue yields nuclear DNA; a telogen (shed) root gives mainly mitochondrial DNA.
• Limitation: microscopic comparison is class-level only; individualisation needs DNA.

B. Fibre

Fibres shed during contact and place a suspect at a scene by matching a known textile source.

• Classes: natural (cotton, wool, silk) and man-made (nylon, polyester, acrylic).
• Comparison points: colour and dye composition, diameter, cross-sectional shape, delustrant, refractive index.
• Analysis: polarising microscopy, microspectrophotometry for colour, FTIR for polymer type.
• Significance: value rises with rarity of dye and fibre type and with the number of matching fibres transferred.

C. Glass

Fractured glass produces fragments and patterns that both associate sources and reconstruct events.

• Comparison properties: refractive index (measured by the oil-immersion/Becke line method), density, and elemental composition.
• Density check: a fragment suspended in a liquid of matching density neither sinks nor floats, confirming equal density.
• Fracture analysis:
  • 3R rule: Radial cracks are stressed on the Reverse (opposite) side of the impact force.
  • Concentric fractures form on the same side as the force.
  • Sequence: backward-fracture and concentric ridge (hackle) marks reveal which impact came first — useful for bullet-hole order.
• Individualisation: a physical fracture (jigsaw) match between two pieces is individualising.

D. Tool marks

Tool marks are impressions or striations left when a harder tool acts on a softer surface, potentially identifying a specific tool.

• Types:
  1. Impression marks: the tool's shape pressed into a surface (e.g., pliers, hammer) — largely class information about tool type and size.
  2. Striation marks: parallel scratches from a sliding edge (e.g., screwdriver prying) — carry individual detail from micro-imperfections on the edge.
• Comparison: test marks made with the suspect tool in soft material (wax, lead) are compared under a comparison microscope.
• Recovery: photograph and, where the object cannot be moved, cast the mark with silicone or dental-type material; never fit the suspect tool into the mark, as that alters it.
• Value: aligned striations across the questioned and test marks can individualise a tool to the exclusion of others.