Criminalistics (Forensic Science)
Domain 6 - Criminalistics (Forensic Science) applies physical sciences to legal questions: identifying, collecting, preserving, and interpreting evidence.
Core rules
- The crime scene is secured, documented, searched, and processed in a sequence. Locard's exchange principle holds that contact leaves traces. Chain of custody records every transfer; a break can destroy evidentiary value.
- Fingerprints may be latent, patent, or plastic. Examiners compare ridge characteristics and report identification, exclusion, or inconclusive. DNA requires proper collection, contamination control, and statistical interpretation. Ballistics compares class and individual characteristics of bullets and cartridge cases. Questioned documents examine handwriting, ink, paper, and alterations. Toxicology and serology identify poisons and biological fluids.
- Forensic results are not self-proving. The expert must be qualified, the method reliable, and the evidence authenticated. Presumptive tests screen; confirmatory tests establish.
Worked example
A cartridge case is recovered at a shooting scene. The lab test-fires the suspect's pistol and compares breech-face, firing-pin, and extractor marks. If class characteristics match but individual marks are inconclusive, the examiner cannot say the pistol fired the case. The report should say inconclusive, and the prosecution must corroborate with other evidence. A matching bullet does not by itself prove who pulled the trigger.
Traps
Do not confuse class characteristics with individual identification. Do not treat a field drug test or gunshot residue as conclusive. Do not assume a fingerprint match without verified comparison. Do not let unauthorized persons enter the scene or handle evidence without documentation.