Criminal Investigation and Evidence Handling
What this domain tests
How a case is built and preserved: lawful collection, defensible packaging, chain of custody, identification procedures, and disclosure obligations.
Core rules
- Locard's exchange principle: every contact leaves a trace. Document what you touched, when, and with what.
- Chain of custody means an unbroken, documented record of who had the item and when. Every transfer needs a name, date, and time.
- Packaging: biological evidence air-dried and in paper, never sealed wet in plastic; firearms unloaded, rendered safe, and secured with ammunition packaged separately; drugs weighed, field-tested, and sealed with initials across the seal.
- Miranda applies to custodial interrogation, not to general on-scene questions or booking.
- Eyewitness identification should use a double-blind, sequential procedure with a non-suggestive filler set.
- Digital evidence requires a warrant for phone contents; preserve with a forensic download, not by scrolling through the device.
- Brady requires disclosure of exculpatory and impeachment material to the prosecutor.
Worked example
A revolver is recovered from a suspect's waistband. The officer photographs it in place, unloads it, records the serial number, swabs for DNA before handling the grip excessively, packages the cartridges separately, seals the gun in a box with initials, date, and case number across the seal, and logs each transfer into property.
Traps
- Bare-handing items that need prints or DNA.
- Sealing wet blood in plastic, which destroys the sample.
- Testifying that an item is the same one seized without being able to show the chain.
- Suggestive showups that taint later identifications.
- Failing to document what was not collected and why.