Obtaining and Identifying Physical Evidence. A large part of an investigator’s role centers around obtaining information and evidence—proof that a crime has been committed as well as proof that a particular person (suspect) committed the crime. Locard’s Principle of Exchange, developed in 1928, states that when any two objects come into contact, a transfer of material from each object to the other always occurs. Investigators use such material to build connections between objects, people and locations. All important decisions will revolve around the available evidence and how it was obtained. Some agencies have established the position of evidence technician. The evidence technician is usually a patrol officer who has received extensive classroom and laboratory training in crime scene investigation. In departments that have small detective bureaus and relatively inexperienced officers, this position fills a notable void. The officer has not been relieved of regular patrol duties but may be called on to conduct crime scene investigations. The kind of evidence to be anticipated is often directly related to the type of crime committed. Scenes of violent crimes frequently contain such evidence as blood, hair, fibers, fingerprints and weapons. Scenes of property crimes are commonly characterized by forcible entry with tools leaving marks on doors, windows, safes, money chests, cash registers and desk drawers. Among the most common types of evidence found at the crime scene are fingerprints, blood, hair, fibers, documents, footprints or tire prints, tool fragments, tool marks, broken glass, paint, insulation from safes, bite marks, firearms and explosives. Fingerprints are often found at crime scenes. According