Criminal Investigations and Evidence Handling
Core concepts
Investigation begins with scene preservation. First responders secure the scene, render aid, identify witnesses, and prevent contamination. Evidence handling requires a documented chain of custody from collection to court. Package items separately, label with date, time, collector, location, and case number, and seal containers. Use proper packaging for biological evidence, sharps, digital devices, and firearms. Photograph before moving items and document measurements. Interviews should be voluntary, documented, and free of coercion. Know Brady obligations: exculpatory evidence must be disclosed. Digital evidence requires preservation, hash verification, and lawful access. Eyewitness identification procedures should be fair and documented; avoid suggestive showups when alternatives exist.
Traps
Questions often test contamination: one package for multiple items, improper drying of wet biological evidence, or undocumented transfers. Do not process evidence at the scene if it should be collected by a specialist. Do not confuse chain of custody with proof of guilt. Do not promise leniency to obtain a statement. Do not ignore evidence that hurts your case.
Worked example
You recover a bloodstained shirt. Photograph it in place, package it in paper after air-drying, seal and initial the package, and record every transfer. Rule of thumb: if you cannot explain every hand that touched the evidence, the chain has a gap. Document the gap immediately.