Investigation and Evidence
Investigation and Evidence
This domain tests crime-scene management, evidence collection and continuity, interviews, eyewitness identification, and the legal duties that govern investigations. The exam rewards investigators who can explain what they did, why they did it, and how the evidence stayed reliable.
Core rules
- Locard's exchange principle: every contact leaves a trace; protect the scene before collecting.
- Chain of custody: document each transfer by name, date, time, purpose, and seal condition. Gaps create doubt.
- Types of evidence: testimonial, physical, documentary, digital, and demonstrative. Each has different foundation requirements.
- Interviews vs interrogations: interviews gather information; interrogations are accusatory and may require Miranda if custodial.
- Eyewitness identification: use unbiased procedures, avoid suggestive comments, and document the witness's own words.
- Brady: disclose exculpatory and impeachment evidence.
Worked example. An officer finds a cartridge case, places it in an envelope, initials the seal, and logs it. The officer then leaves the envelope overnight in a patrol car and transfers it the next day. At trial, the defense challenges continuity. Reasoning: the item is relevant, but the chain has a gap and possible contamination. The officer should have sealed, labeled, and submitted it promptly, with each transfer documented.
Traps
- Allowing unnecessary people into the scene.
- Asking leading questions that shape a witness's account.
- Showing a single photo or suspect in a suggestive way.
- Writing conclusions such as "the suspect was nervous" without observable facts.
- Failing to document exculpatory statements or evidence.
Exam focus
State the investigative step, the legal or scientific reason for it, and how you preserved reliability.