Criminal Investigation and Evidence Collection
What this domain tests
This domain tests scene management, evidence handling, interview and interrogation rules, and eyewitness identification law.
Core rules
- Locard's exchange principle: every contact leaves a trace.
- Evidence types: testimonial, physical, direct, circumstantial, and demonstrative. All physical evidence needs authentication and a chain of custody.
- Scene sequence: render aid and secure, establish a scene log, photograph before moving, sketch and measure, collect, package, seal, label, and document each transfer.
- Chain of custody must show every person who handled the item, when, and why, with no unexplained gaps.
- Custodial interrogations of suspects in homicide and listed serious offenses must be electronically recorded (725 ILCS 5/103-2.1), with narrow exceptions documented.
- Lineups in Illinois are governed by 725 ILCS 5/107A-2: blind or blinded administration, unbiased instructions, and fillers matching the description. Showups require immediate proximity and probable cause; photo arrays should follow the same non-suggestive principles.
Worked example
An officer recovers a shell casing. She photographs it in place, notes the location on the sketch, picks it up with gloves, places it in a paper envelope (breathable), seals it, initials across the seal, logs the time, and delivers it to the property room where a property officer signs it in. Testimony at trial can recite each link without a gap.
Traps
Packaging damp biological evidence in plastic, which promotes degradation. Failing to document the moment of collection. Using a suggestive showup when a lineup is feasible. Forgetting that a suspect's statements during a recorded interview must be captured from the start, not after a preliminary chat.