The concept known as "Locard's Exchange Principle" states that every time someone enters an environment, something is added to and removed from the scene. The principle is sometimes stated as "every contact leaves a trace," and applies to contact between individuals as well as between individuals and a physical environment. Law enforcement investigators should always assume that physical evidence is left behind at every scene.
The amount and nature of the physical evidence deposited will be largely dependent on the circumstances of the crime and may include:
- Trace evidence — fibers, hair, glass, paint chips.
- Biological evidence — blood, saliva, semen.
- Comparative evidence — latent fingerprints, tool marks.
Items of physical evidence are not always visible to the naked eye and may be easily overlooked. A methodical approach to the collection and preservation of evidence is essential. One exception is that if evidence integrity is at risk, it is important to make rapid decisions to prevent degradation or loss. Agency protocol dictates who collects this type of evidence.
Additional Online Courses
- What Every First Responding Officer Should Know About DNA Evidence
- Collecting DNA Evidence at Property Crime Scenes
- DNA – A Prosecutor’s Practice Notebook
- Crime Scene and DNA Basics
- Laboratory Safety Programs
- DNA Amplification
- Population Genetics and Statistics
- Non-STR DNA Markers: SNPs, Y-STRs, LCN and mtDNA
- Firearms Examiner Training
- Forensic DNA Education for Law Enforcement Decisionmakers
- What Every Investigator and Evidence Technician Should Know About DNA Evidence
- Principles of Forensic DNA for Officers of the Court
- Law 101: Legal Guide for the Forensic Expert
- Laboratory Orientation and Testing of Body Fluids and Tissues
- DNA Extraction and Quantitation
- STR Data Analysis and Interpretation
- Communication Skills, Report Writing, and Courtroom Testimony
- Español for Law Enforcement
- Amplified DNA Product Separation for Forensic Analysts