Crime Scene Investigation
This domain tests how a scene is processed from first response to evidence transfer. The exam emphasizes scene integrity, the order of documentation, contamination control, and a defensible chain of custody rather than memorized definitions.
Core rules and concepts
- First response: preserve life, secure the scene, and limit entry. Start a scene log recording name, agency, time in/out, and reason for entry.
- Documentation order: notes, photographs, and sketch before collection or movement. Photograph the item in place, then mid-range, then close-up with a scale.
- Search patterns: line/strip for open ground, grid for thorough coverage, spiral for compact scenes, zone for large or compartmented areas, wheel for a central point.
- Locard's exchange principle: every contact leaves trace evidence. Collect known standards, substrate controls, and elimination prints where relevant.
- Packaging: air-dry wet biological evidence before sealing in breathable containers; package sharp items in puncture-resistant containers; package each item separately and label with initials, date, and contents.
- Chain of custody: begins at collection and records every transfer, seal, and storage location.
Worked example
A forced-entry burglary scene has blood on a kitchen knife. Photograph the knife in place with a scale and note its orientation. Air-dry the knife, package it in a rigid container, and collect a substrate control from the counter beneath it. Swab the windowsill for touch DNA only if the case theory supports it. Lift a footwear impression separately and document the lift location. Reasoning: drying preserves DNA, the substrate control supports interpretation, and separate packaging prevents cross-contamination.
Traps that cost marks
- Collecting before photographing the item in place.
- Sealing wet blood or tissue in plastic.
- Skipping substrate or elimination controls.
- Over-collecting unrelated items and increasing contamination risk.
- Gaps in the chain-of-custody record or undocumented scene entries.