Criminal Investigation
Core concepts
Know the investigative process: preliminary investigation, securing the scene, identifying and interviewing witnesses, collecting and preserving evidence, chain of custody, and case follow-up. Understand the difference between interview (non-custodial) and interrogation (custodial). Know evidence types (testimonial, physical, documentary, direct and circumstantial) and the value and limits of each. Understand crime-scene documentation (notes, photographs, sketches), evidence packaging to prevent contamination, and the role of forensic analysis. Know the elements of common offenses and how to build a case that meets each element.
Traps
Questions often test chain of custody-every transfer of evidence must be documented, or admissibility may be challenged. Another trap is treating circumstantial evidence as weak; it can be sufficient. Watch for questions that confuse an interview with an interrogation or that assume a confession alone proves a case. Also be alert to contamination risks, such as improper packaging of biological evidence.
Rule of thumb
Build the case element by element. For each offense, list the elements, then map evidence to each element. If an element lacks evidence, the case is incomplete regardless of how strong other evidence is.
Worked example
In a burglary investigation, the officer documents forced entry (element: unlawful entry), identifies the owner's missing property (element: intent to commit a crime therein), and preserves a latent print from the broken window with a documented chain of custody. Each element is supported by specific evidence.