Why must crime-scene investigators carefully photograph, measure, sketch, and catalog evidence?

Crime-scene investigators (CSIs) must meticulously photograph, measure, sketch, and catalog evidence because there is typically only one opportunity to secure evidence from a crime scene. A methodical and complete investigation is essential to ensure that all physical evidence—which may have been touched or microscopically contaminated by a suspect—is properly preserved for analysis.1

These practices are fundamental to criminalistics, which involves the application of scientific methods to recognize, identify, and compare evidence to reconstruct the events of a crime. By documenting the scene through photography, detailed notes, and sketches, investigators create a reliable record that supports the subsequent evaluation of evidence in a crime laboratory. This systematic approach ensures that evidence is accurately tracked, often through unique identifiers or bar codes, which maintains the chain of custody necessary for legal proceedings. Ultimately, these rigorous procedures are vital for the integrity of the criminal justice system, as they allow forensic scientists to analyze materials—ranging from biological fluids to trace evidence—to identify victims and perpetrators.23