Criminal Investigation and Evidence
Investigation Basics
Investigations move from initial response through scene management, evidence collection, follow-up, and case presentation. Understand the difference between direct evidence (eyewitness, confession) and circumstantial evidence (inferences from facts), and know that circumstantial evidence can be sufficient. Learn chain of custody-every transfer of evidence must be documented to preserve integrity-and the categories of evidence: testimonial, documentary, physical, and demonstrative. Know the rules of admissibility: relevance, materiality, and the balance of probative value against prejudice.
Crime Scene Management
Secure, protect, and document the scene before collecting. Understand Locard's exchange principle (every contact leaves a trace), the importance of photographs and sketches, and the order of collection (perishable evidence first). Know interview versus interrogation, and the value of separating witnesses. Understand the role of the case file and the need for accurate, contemporaneous notes.
Traps and Rule of Thumb
Traps include believing circumstantial evidence is weak or inadmissible, breaking chain of custody by undocumented transfers, and interviewing witnesses together so stories contaminate. Another trap is collecting before documenting.
Worked example: An officer finds a weapon at a scene. Correct sequence: photograph in place, note location and condition, wear gloves, package and label, log the transfer, and submit to property. Rule of thumb: document first, collect second, and never let the chain of custody go unwritten.