Unit 2: Physical evidences - Subjective Questions

Physical Evidence refers to any tangible object, material, or substance that can establish a connection between a crime, a victim, a suspect, and the crime scene. It is also known as real evidence or material evidence.

Definition:

• Physical evidence includes any and all objects that can prove that a crime has been committed or can provide a link between a crime and its victim or a crime and its perpetrator.

Significance in Linking a Suspect to a Crime:

• Establishes Corpus Delicti: Proves that a crime has actually occurred.
• Links Suspect and Scene: Through transfer of trace materials (Locard's Exchange Principle).
• Corroborates Testimony: Supports or contradicts witness/suspect statements.
• Identifies Suspect: Fingerprints, DNA, etc. can individualize a person.
• Reconstruction: Helps in reconstructing the sequence of events.

Locard's Exchange Principle states that "every contact leaves a trace" — whenever two objects come into contact, there is an exchange of materials between them, forming the theoretical basis of physical evidence.

Physical evidence can be categorized into numerous types depending on its nature and origin:

Major Types of Physical Evidence:

• Biological Evidence: Blood, semen, saliva, hair, tissue, bones.
• Trace Evidence: Fibers, glass fragments, paint chips, soil, gunshot residue.
• Impression Evidence: Fingerprints, footprints, tire marks, tool marks.
• Firearms Evidence: Weapons, bullets, cartridge cases, gunshot residue.
• Documentary Evidence: Handwriting, forged documents, printed material.
• Drug and Chemical Evidence: Controlled substances, poisons, explosives.
• Digital Evidence: Data from computers, phones, storage devices.

Examples in Practice:

• A broken window glass at a burglary scene (trace/glass).
• A bloodstain pattern on a wall (biological).
• Tool marks on a forced door lock (impression).

Each type requires specific collection, packaging, and preservation methods to maintain its evidentiary value.

Physical evidence is classified in several ways, most importantly by its identifying power:

1. Based on Identifying Ability:

• Individual Evidence (Individualization): Can be attributed to a single, unique source with a high degree of certainty.
  • Examples: Fingerprints, DNA, tool marks matching a specific tool, striations on bullets.
• Class Evidence (Class Characteristics): Can only be associated with a group or class of sources, not a single one.
  • Examples: Blood group, fiber type, glass composition, shoe size.

2. Based on Nature:

• Biological: Blood, hair, tissue.
• Physical/Chemical: Glass, paint, soil, drugs.

3. Based on State:

• Solid, Liquid, and Gaseous evidence.

Key Point:

• The combination of several class characteristics can increase the probative value and approach individualization.
• Individual evidence carries greater weight in court than class evidence.

Physical evidence plays a central and multi-faceted role in both investigation and courtroom proceedings.

Role in Criminal Investigation:

• Proving a Crime Occurred: Establishes the corpus delicti.
• Identifying the Culprit: Links a specific individual to the scene or victim.
• Providing Investigative Leads: Directs the course of investigation.
• Reconstructing the Crime: Determines how events unfolded.
• Eliminating Suspects: Exonerates innocent persons.
• Corroborating or Refuting Statements: Verifies witness/suspect accounts.

Role in Trials:

• Objective and Reliable: Unlike eyewitness testimony, physical evidence is unbiased.
• Scientific Backing: Supported by expert analysis and forensic reports.
• Corroborative Value: Strengthens the prosecution or defense case.
• Admissibility: When properly collected and preserved with an intact chain of custody, it is admissible in court.
• Persuasive Power: Physical evidence is often more convincing to a jury/judge than testimonial evidence.

Note: The value of physical evidence in court depends heavily on the integrity of collection, preservation, and chain of custody.

Systematic search methods ensure that no evidence is overlooked at a crime scene. The choice of method depends on the size, location, and terrain of the scene.

Common Search Patterns:

• Spiral (Inward/Outward) Search:

  • Searcher moves in a spiral either from the center outward or from the periphery inward.
  • Best for small, confined outdoor scenes with a single searcher.

• Strip/Line Search:

  • The area is divided into strips/lanes; searchers walk in straight parallel lines.
  • Suitable for large outdoor areas.

• Grid Search:

  • A double-line search — the strip search performed twice at right angles.
  • Offers thorough, overlapping coverage and reduces chances of missing evidence.

• Zone/Quadrant Search:

  • The scene is divided into zones/sectors, each assigned to a searcher.
  • Ideal for indoor scenes (rooms) or complex areas.

• Wheel/Ray (Pie) Search:

  • Searchers move from a central point outward along radii, like spokes of a wheel.
  • Suitable for small circular scenes.

Key Considerations:

• Search should proceed from general to specific.
• Photograph and document evidence before collection.
• Maintain safety and avoid contamination.

Processing physical evidence involves a systematic, step-by-step procedure to ensure integrity and evidentiary value.

Steps in Processing Physical Evidence:

1. Securing and Isolating the Scene: Prevent contamination and unauthorized access.
2. Recognition: Identifying potential evidence at the scene.
3. Documentation:
   • Photography (overall, mid-range, close-up with scale).
   • Sketching (rough and finished sketches).
   • Note-taking describing location and condition.
4. Collection: Using appropriate tools (forceps, swabs) and wearing gloves.
5. Packaging:
   • Use suitable containers (paper bags for biological, druggist folds for trace).
   • Avoid plastic for wet/biological samples.
6. Labeling and Sealing: Each item marked with case number, date, collector's initials.
7. Transportation: Safe transfer to the laboratory.

Examination in the Laboratory:

• Physical Examination: Size, shape, color, weight.
• Chemical Analysis: Spectroscopy, chromatography.
• Microscopic Examination: Comparison microscopy, SEM.
• Comparison with Standards/Controls.

Goal: To derive maximum information while preserving the evidence for court presentation.

Probative Value refers to the ability of a piece of evidence to prove or establish a fact relevant to the case. It indicates how much the evidence contributes to proving or disproving a matter in question.

Definition:

• Probative value measures the weight and significance of evidence in establishing guilt or innocence.

Factors Affecting Probative Value:

• Individual vs Class Characteristics: Individual evidence (e.g., DNA, fingerprints) has higher probative value than class evidence.
• Rarity of Evidence: Rarer characteristics increase value.
• Integrity of Chain of Custody: Broken chain reduces or nullifies value.
• Proper Collection and Preservation: Contaminated or degraded evidence loses value.
• Relevance to the Case: Direct connection to the crime enhances value.
• Scientific Reliability: Evidence backed by validated methods carries more weight.
• Corroboration: Multiple pieces supporting each other increase overall value.

Example: A single fiber has low probative value, but combined with DNA, fingerprints, and glass fragments, the cumulative probative value becomes very strong.

Conclusion: High probative value evidence is more persuasive and reliable in establishing facts during trial.

Chain of Custody is the chronological documentation showing the seizure, custody, control, transfer, analysis, and disposition of physical evidence.

Definition:

• It is a paper trail that records every person who handled the evidence, and the time, place, and purpose of each transfer, from collection to court presentation.

Key Elements Documented:

• Who collected and handled the evidence.
• What the evidence is (description).
• When it was collected and transferred.
• Where it was collected and stored.
• How it was handled and preserved.

Importance for Admissibility:

• Ensures Integrity: Proves the evidence has not been altered, tampered with, or substituted.
• Establishes Authenticity: Confirms the evidence presented is the same as collected.
• Legal Requirement: Courts reject evidence with a broken or unaccounted chain of custody.
• Prevents Contamination Claims: Defends against allegations of mishandling.
• Maintains Credibility: Strengthens the reliability of forensic findings.

Consequence of Break: A broken chain of custody can render even the most incriminating evidence inadmissible, potentially collapsing the case.

Hair is one of the most commonly encountered types of trace evidence at crime scenes.

Structure of Hair:
A hair shaft consists of three main layers:

• Cuticle: Outermost scaly layer; scale patterns help species identification.
• Cortex: Middle layer containing pigment granules; important for color comparison.
• Medulla: Central core; the medullary index helps distinguish human from animal hair.

Forensic Significance:

• Hair is easily transferred and resistant to decomposition.
• Follows Locard's Exchange Principle — transferred during contact.

Information Obtained from Hair Examination:

• Human vs Animal Origin: Based on medulla, scale pattern, pigmentation.
• Body Area Origin: Head, pubic, facial, limb hair.
• Racial/Ancestral Origin: From cross-sectional shape and pigment distribution.
• Manner of Removal: Forcibly pulled (root sheath present) vs naturally shed.
• Presence of Damage/Disease: Bleaching, dyeing, cutting.
• Toxicological Analysis: Drugs, poisons (e.g., arsenic) can be detected.
• DNA Analysis: Nuclear DNA from root; mitochondrial DNA from shaft.

Limitation: Microscopic hair comparison provides class evidence; individualization requires DNA analysis.

Fibres are among the most common forms of trace evidence, transferred readily during physical contact.

Classification of Fibres:

• Natural Fibres:

  • Animal origin: Wool, silk (protein-based).
  • Plant origin: Cotton, jute, linen (cellulose-based).
  • Mineral origin: Asbestos.

• Synthetic/Man-made Fibres:

  • Nylon, polyester, acrylic, rayon.

• Regenerated Fibres:

  • Rayon, acetate (chemically processed natural materials).

Forensic Significance:

• Fibres transfer during contact between clothing, carpets, upholstery, etc.
• Their presence links a suspect, victim, and scene.

Methods of Examination:

• Visual and Microscopic Examination: Color, diameter, cross-section, surface features using comparison microscopy.
• Polarized Light Microscopy: Determines refractive index and birefringence.
• Microspectrophotometry: Compares dye color composition.
• Chromatography (TLC): Analyzes dye components.
• Infrared Spectroscopy (FTIR): Identifies chemical composition of synthetic fibres.
• Melting Point Determination: Distinguishes synthetic fibres.

Evidentiary Value: Fibres are typically class evidence; matching multiple properties enhances significance.

Glass is a frequently encountered form of physical evidence in cases involving burglaries, hit-and-run accidents, and assaults.

Physical Properties Used for Comparison:

• Density: Determined using the flotation method or density gradient tubes.
• Refractive Index (RI): The most important property, measured using the immersion (Becke line) method. Different glasses have characteristic RIs.
• Color, Thickness, and Surface Features.
• Chemical Composition: Trace elements analyzed by SEM-EDX, ICP-MS.

Fracture Pattern Analysis:

• Radial Cracks: Extend outward from the point of impact like spokes.
• Concentric Cracks: Form circles around the impact point.
• 3R Rule: Radial cracks form Right angles on the Reverse (opposite) side of the force — helps determine the direction of impact.
• Sequence of Impacts: Determined by the way fractures terminate at existing cracks.
• High vs Low Velocity Impact: Bullet holes show cone-shaped (crater) exits, indicating direction of travel.

Forensic Significance:

• Determines direction and sequence of breakage.
• Links glass fragments on a suspect to a broken window (transfer).

Evidentiary Value: Physically fitting fragments (jigsaw fit) provides individualizing evidence, while RI/density comparison gives class evidence.

Tool marks are impressions, scratches, or abrasions produced when a tool comes into contact with a surface.

Definition:

• A tool mark is any impression, cut, gouge, or abrasion made when a tool is brought into contact with another object.

Types of Tool Marks:

• Impression/Indentation Marks: Produced by pressing a tool against a softer surface (e.g., a hammer blow, pry bar on a door frame). Reflects the shape of the tool.
• Striated/Abrasion Marks: Produced when a tool slides or scrapes across a surface, leaving parallel scratch lines (e.g., screwdriver, cutting pliers). Reflect surface irregularities of the tool edge.
• Combination Marks: Both compression and striation (e.g., bolt cutters).

Examination and Comparison Process:

1. Documentation: Photograph and measure the mark at the scene.
2. Casting: Use silicone/dental material for indentation marks.
3. Preparation of Test Marks: Suspected tool used on similar test material at various angles.
4. Comparison Microscopy: The evidence mark and test mark are compared side by side.
5. Class Characteristics: Type, size, shape of tool.
6. Individual Characteristics: Microscopic imperfections (nicks, wear) unique to the tool.

Evidentiary Value:

• When individual characteristics (unique striations) match, tool marks can individualize a specific tool, offering high probative value.

Understanding the difference between individual and class characteristics is fundamental to evaluating the evidentiary value of physical evidence.

Key Points:

• Individual evidence results from random, unique events (e.g., wear on a tool, ridge patterns).
• Class evidence results from manufacturing or biological categories.
• Combining multiple class characteristics increases discriminating power and can approach individualization.

Example: A blue polyester fiber (class) found on a victim gains significance when combined with a matching DNA profile (individual), strengthening the overall case.

Correct collection, packaging, and preservation are essential to maintain the integrity and evidentiary value of physical evidence.

General Principles:

• Wear gloves and use clean tools to avoid contamination.
• Package each item separately to prevent cross-contamination.
• Label and seal every container with case details.

Type-Specific Methods:

• Biological Evidence (blood, semen, tissue):

  • Dry before packaging; use paper bags/envelopes, never plastic (retains moisture, promotes bacterial growth).
  • Refrigerate or freeze if required.

• Hair and Fibres:

  • Collect with forceps; place in paper folds (druggist fold) or envelopes.

• Glass:

  • Wrap in paper/cotton, place in a rigid box to avoid further breakage.

• Tool Marks:

  • Collect the entire object if possible; otherwise cast using silicone material.

• Liquids and Chemicals:

  • Use airtight, non-reactive containers (glass vials).

• Firearms:

  • Handle carefully; do not clean the barrel; package unloaded.

Preservation Considerations:

• Maintain proper temperature and humidity.
• Avoid exposure to light, heat, and moisture.
• Document with chain of custody at every step.

Both grid and zone searches are systematic techniques used to locate physical evidence, but they differ in application and coverage.

Grid Search:

• Method: A strip/line search conducted twice, the second at right angles (perpendicular) to the first, forming a grid.
• Coverage: Double, overlapping coverage — very thorough.
• Best For: Large outdoor areas where high thoroughness is required.
• Advantage: Minimizes chances of missing evidence.
• Disadvantage: Time-consuming and labor-intensive.

Zone (Quadrant) Search:

• Method: The scene is divided into sectors/zones, each assigned to one or more searchers.
• Coverage: Each zone searched independently, sometimes with sub-division.
• Best For: Indoor scenes, rooms, or complex/large scenes.
• Advantage: Allows division of labor; manageable for teams.
• Disadvantage: Coverage depends on how zones are defined.

Comparison Table:

Conclusion: Grid search offers superior thoroughness, while zone search offers better organization for complex or indoor scenes.

Locard's Exchange Principle is the foundational theory of trace/physical evidence in forensic science.

Statement of the Principle:

• "Every contact leaves a trace." — Proposed by Dr. Edmond Locard, it states that whenever two objects or persons come into contact, there is a mutual exchange of materials between them.

Explanation:

• The perpetrator will bring something into the scene and leave with something from it.
• Both the trace they leave and the trace they take can serve as forensic evidence.

Significance in Investigation:

• Basis of Trace Evidence: Justifies collection of hair, fibres, soil, glass, etc.
• Links Suspect and Scene: Transfer of materials establishes connections between suspect, victim, and location.
• Guides Search Efforts: Investigators know that contact points hold evidence.
• Supports Reconstruction: Transferred materials help rebuild the sequence of events.

Examples:

• Fibres from a suspect's clothing found on the victim.
• Soil from the crime scene found on a suspect's shoes.
• Glass fragments from a broken window embedded in a burglar's clothing.

Conclusion: Locard's Principle underlies the entire concept of physical/trace evidence, making meticulous crime scene processing essential.

Glass fracture analysis is a powerful tool for reconstructing events involving broken glass.

Types of Fractures:

• Radial Fractures: Cracks that radiate outward from the point of impact, like spokes of a wheel. They form on the side opposite to the force first.
• Concentric Fractures: Cracks that form roughly circular patterns around the point of impact.

Determining Direction of Force (3R Rule):

• 3R Rule: Radial cracks form a Right angle on the Reverse side of the force.
• By examining stress marks (rib marks/conchoidal lines) on the edge of a radial crack:
  • The stress marks are perpendicular to the surface where the force was applied.
  • They are parallel to the opposite surface.
• This reveals from which side the force originated.

Determining Sequence of Impacts:

• When multiple impacts occur, a new fracture line terminates when it meets a pre-existing crack.
• Applying the principle: a fracture ends at an earlier fracture. By tracing which cracks stop at others, the order of impacts is established.

Bullet Holes:

• Produce a cone-shaped (crater) hole, wider on the exit side, indicating the direction of travel.

Conclusion: Radial and concentric fractures, combined with rib marks and the 3R rule, allow reconstruction of both the direction and sequence of glass breakage.

Distinguishing human from animal hair is a primary step in forensic hair analysis.

Features Used for Differentiation:

1. Medulla:

• Human Hair: Medulla is absent, fragmentary, or narrow (amorphous).
• Animal Hair: Medulla is prominent, continuous, and often patterned (ladder-like, cellular).

2. Medullary Index:

• Defined as the ratio of the diameter of the medulla to the diameter of the whole hair:
  
• Human Hair: Medullary Index is less than 0.3 (usually < 1/3).
• Animal Hair: Medullary Index is greater than 0.5 (usually > 1/2).

3. Cuticle Scale Pattern:

• Human: Flattened, imbricate (overlapping) scales.
• Animal: Coronal (crown-like) or spinous patterns.

4. Pigment Distribution (Cortex):

• Human: Pigment evenly distributed toward the cuticle.
• Animal: Pigment concentrated centrally (toward the medulla).

5. Root Shape:

• Human roots differ from the brush-like or elongated animal roots.

Conclusion: The medullary index is a key quantitative parameter — a low index indicates human origin, while a high index indicates animal origin.

Documentation is a critical stage of crime scene processing that permanently records the scene and evidence before anything is disturbed.

Three Main Forms of Documentation:

1. Photography:

• Captures the scene in its original, undisturbed state.
• Types of shots:
  • Overall photographs: General view of the scene.
  • Mid-range photographs: Relationship of evidence to surroundings.
  • Close-up photographs: Detailed images, taken with and without a scale.
• Provides visual proof admissible in court.

2. Sketching:

• Rough Sketch: Drawn at the scene with measurements.
• Finished Sketch: Prepared later, to scale.
• Shows exact locations, distances, and spatial relationships of evidence that photographs may distort.

3. Notes:

• Written record describing the condition, location, and details of each item.
• Includes date, time, weather, personnel present, and observations.

Importance:

• Preserves the Scene: Creates a permanent, objective record.
• Supports Reconstruction: Helps recreate the scene later.
• Court Admissibility: Corroborates testimony and evidence.
• Maintains Chain of Custody: Documents original placement.
• Refreshes Memory: Aids investigators and experts during trial.

Golden Rule: Document first, collect later — evidence must be recorded in place before being moved.

Forensic laboratories employ a range of scientific techniques to analyze physical evidence and extract meaningful information.

Major Laboratory Techniques:

1. Microscopy:

• Comparison Microscope: Side-by-side comparison of hairs, fibres, bullets, tool marks.
• Polarized Light Microscopy: Fibre and mineral identification.
• Scanning Electron Microscope (SEM-EDX): Gunshot residue, glass, elemental analysis.

2. Spectroscopy:

• UV-Visible Spectrophotometry: Color/dye analysis.
• Infrared (FTIR) Spectroscopy: Identification of synthetic fibres, paints, polymers.
• Atomic Absorption Spectroscopy (AAS): Metal/trace element detection.

3. Chromatography:

• Thin Layer Chromatography (TLC): Dye and ink separation.
• Gas Chromatography (GC), GC-MS: Drugs, accelerants, poisons.
• HPLC: Non-volatile compounds.

4. Physical Methods:

• Refractive Index and Density Measurement: Glass comparison.
• Melting Point: Fibre identification.

5. DNA Analysis:

• PCR and STR profiling: Individualization of biological evidence.

Applications Summary:

• Trace evidence → microscopy, spectroscopy.
• Chemical/drug evidence → chromatography.
• Biological evidence → DNA analysis.
• Elemental analysis → SEM-EDX, ICP-MS.

Conclusion: The choice of technique depends on the nature of the evidence, ensuring accurate, reproducible, and court-admissible results.