Comprehensive Forensic Science and Crime Scene Investigation Study Guide
Principles of Forensic Science
- Dr. Edmond Locard:
- Recognized as the father of modern forensics.
- Formulated Locard's Exchange Principle, which serves as a major foundation of modern forensic investigation.
- Locard's Exchange Principle: States that whenever two objects come into contact with one another, they exchange material or leave traces behind.
The Seven Steps of Crime Scene Investigation
- Crime scene investigation follows a structured, seven-step sequence:
- Securing the Scene: First responders isolate and protect the physical area to avoid loss or contamination of evidence.
- Separating Witnesses: Witnesses are physically separated to ensure independent account collection.
- Scanning the Scene: Forensic teams assess the scene visually to identify primary and secondary scenes and determine photo priorities.
- Seeing the Scene: The scene is formally documented via comprehensive photography (overall, mid-range, and close-up views).
- Sketching the Scene: Accurate rough and finalized sketches are created, depicting precise spatial relationships and physical dimensions.
- Searching the Scene: Investigators conduct systematic, patterned searches to identify all physical and biological trace evidence.
- Securing and Collecting Evidence: Evidence is properly packaged, labeled, sealed, and cataloged for transport to the crime laboratory.
Crime Scene Management and First Responder Duties
- First Responder Duties:
- When a first responder secures a crime scene, their primary duties are to:
- Protect and isolate the area.
- Establish a clear physical boundary.
- Control and restrict who enters and exits the scene.
- Prevent evidence from being contaminated, altered, or disturbed.
- Crime Scene Documentation:
- Crime Scene Log: The official document that logs every person who enters or exits the crime scene, recording their identity and timing.
- Evidence Markers: Specific visual markers placed next to physical items during two distinct stages of CSI:
- Photographing or documenting the scene.
- Collecting the evidence.
- Witness Management Protocols:
- Eyewitnesses must be separated immediately before they are interviewed.
- Purpose of Separation: Prevents witnesses from influencing each other's memories, cross-contaminating stories, or altering their statements based on peer feedback.
Evidence Collection, Packaging, and Preservation Protocols
- Collection of Wet Evidence:
- Wet evidence must be completely air-dried first before being placed into a permanent storage container.
- Packaging Rule: Wet evidence should not be sealed immediately in airtight containers (such as plastic bags) because accumulated moisture causes degradation and mold growth, damaging biological samples.
- Collection of Small, Dry Evidence:
- Small, dry physical items (e.g., hair strands, fibers, powders, or nail clippings) must be placed inside a paper bundle (also called a druggist fold or bindle).
- Once secured inside the paper bundle, the entire bundle is placed inside a standard evidence envelope.
- Bindle: A folded piece of paper specifically folded to contain, insulate, and protect small trace evidence from leaking or becoming contaminated.
Chain of Custody and Evidence Handling Procedures
- Importance of Chain of Custody:
- A mandatory paper trail that tracks every individual who handled the evidence, the exact timestamp of handling, and where the evidence was stored or transferred.
- Ensures that evidence remains verifiable, reliable, and unadulterated.
- Vital for maintaining the legal admissibility of evidence in court.
- Procedure for Opening Sealed Evidence Bags:
- When an investigator receives bagged evidence that has already been sealed and signed across the tape label, they must follow a strict opening protocol:
- Do not open or cut through the original sealed edge.
- Cut open the evidence bag from a completely different, unsealed area or edge.
- Remove and analyze the evidence.
- Return the evidence to the bag, reseal the new cut edge with evidence tape, and write their initials and date across the new seal.
Categorization and Classification of Forensic Evidence
- Direct Evidence vs. Circumstantial Evidence:
- Direct Evidence: Evidence that directly proves an alleged fact without requiring any inference or presumption.
- Examples of Direct Evidence:
- A written confession signed by a suspect.
- A tape recording of a suspect admitting to a crime.
- Circumstantial Evidence: Indirect evidence that relies on an inference or logical deduction to connect it to a conclusion of fact.
- Examples of Circumstantial Evidence:
- A fingerprint left on a door handle of a car.
- A fingerprint left on a soda can found at a crime scene.
- A piece of gum left at a crime scene.
- Physical Evidence vs. Biological Evidence:
- Physical Evidence: Non-living physical materials or synthetic objects retrieved from a crime scene.
- Examples of Physical Evidence:
- Jacket fiber (e.g., 100% cotton).
- Bullet casing.
- Glass fragments.
- Ransom note.
- Biological Evidence: Organic material originating from a living organism.
- Examples of Biological Evidence:
- Blood, saliva, skin cells, hair, or other bodily materials.
- Individual Evidence vs. Class Evidence:
- Individual Evidence: Evidence with unique attributes that can be traced back to one specific, singular source or individual to the exclusion of all others.
- Examples of Individual Evidence:
- Skin cells found under the fingernails of a victim (analyzed for DNA).
- A fingerprint left on a soda can found at a crime scene.
- Class Evidence: Evidence that can only be linked to a broad group, category, or batch of manufactured items or individuals, but not to a unique single source.
- Examples of Class Evidence:
- Size 9 Nike shoe found beside a dead body.
- A 1990 Ford Explorer seen driving away from an armed robbery.