Physical evidence must be handled and processed in a way that prevents any change from taking place between the time it is removed from the crime scene and the time it is received by the crime laboratory. Changes can arise through contamination, breakage, evaporation, accidental scratching or bending, or loss through improper or careless packaging.
The integrity of evidence is best maintained when the item is kept in its original condition as found at the crime site. Whenever possible, evidence should be submitted to the laboratory intact. Blood, hairs, fibers, soil particles, and other types of trace evidence should not normally be removed from garments, weapons, or other articles that bear them. Instead, the entire object is to be sent to the laboratory for processing. Of course, if evidence is adhering to an object in a precarious manner, removing and packaging the item best exercise good judgment. If evidence is found adhering to large structures, such as a door, wall, or floor, common sense must be used; remove the specimen with a forceps or other appropriate forensic tool. In the case of a bloodstain, one has the option of either scraping the stain off the surface, transferring the stain to a moistened swab, or cutting out the area of the object bearing the stain.
Each different item or similar items collected at different locations must be placed in separate containers. Packaging evidence separately prevents damage through contact and prevents cross-contamination.
The well-prepared evidence collector will arrive at a crime scene with a large assortment of packaging materials and tools ready to encounter any type of situation. Forceps and similar tools may have to be used to pick up small items. Unbreakable plastic pill bottles with pressure lids are excellent containers for hairs, glass, fibers, and various other kinds of small or trace evidence. Alternatively, envelopes, screw-cap glass vials, or cardboard pillboxes are adequate containers for most trace evidence encountered at crime sites. Ordinary mailing envelopes should not be used as evidence containers because powders and fine particles will leak out of their corners. Small amounts of trace evidence can also be conveniently packaged in a carefully folded paper, using what is known as a “druggist fold”. This consists of folding one end of the paper over one-third, then folding the other end (one-third) over that, and repeating the process from the other two sides. After being folded in this manner, the outside two edges are tucked into each other to produce a closed container that keeps the specimen from falling out. Although pill bottles, vials, pillboxes, or manila envelopes are good universal containers for most trace evidence for forensic document examination, two frequent finds at crime scenes warrant special attention. If bloodstained materials are stored in airtight containers, the accumulation of moisture may encourage the growth of mold, which can destroy the evidential value of blood. In these instances, wrapping paper, manila envelopes, or paper bags are recommended packaging materials. As a matter of routine, all items of clothing are to be air-dried and placed individually in separate paper bags to ensure a constant circulation of air through them. This will prevent the formation of mold and mildew. On the other hand, charred debris recovered from the scene of a suspicious fire must be sealed in an airtight container to prevent the evaporation of volatile petroleum residues. New paint cans or tightly sealed jars are recommended in such situations.
