Crime Detection and Investigation
Domain 3 - Crime Detection and Investigation tests the legal and technical boundaries of building a case: lawful collection, custodial rights, chain of custody, and presentation in court.
Core rules
- Investigation is a logical process: gather facts, identify and interview witnesses, collect and preserve evidence, develop suspects, and prepare the case for prosecution. The investigator must distinguish facts from inference and document every step.
- Custodial investigation begins when a person is taken into custody and questioned about a crime. The rights to remain silent, to counsel, and to be informed of those rights must be given. Waiver must be voluntary, in writing, and made with counsel.
- Entrapment is lawful: officers create an opportunity for a person already disposed to commit a crime. Inducement or instigation is not lawful: officers plant the criminal idea and pressure an otherwise innocent person. The distinction matters to criminal liability.
- For dangerous drugs, chain of custody requires marking, physical inventory, photograph, and presence of the accused or required witnesses at the seizure point. Every transfer must be documented. Gaps create reasonable doubt.
Worked example
In a buy-bust, the poseur-buyer hands marked money to the suspect, who gives a sachet. If officers arrest the suspect but do not mark the sachet at the scene and conduct no inventory, the defense will attack identity and integrity. The prosecution must explain every link: seizure, marking, turnover to the laboratory, testing, and court presentation. A conviction cannot rest on a presumptive field test alone.
Traps
Miranda does not apply to every question asked in public; it applies to custodial interrogation. Spontaneous statements are not automatically excluded. Inquest is not a full preliminary investigation. Do not confuse entrapment with instigation. Do not treat a delay in turnover as harmless without explanation.