Processing

It is of critical importance to properly identify, collect, preserve, and transport forensic scientific evidence for processing. During the investigation of a crime, the initial objectives regarding evidence are to thoroughly document and photograph the scene and to annotate the description and location of evidence to be gathered. A systematic process is then used to collect and package evidence for transport to the laboratory. Photographing may continue throughout the sample collection process, particularly if there are multiple layers of evidence that can only be seen as those above them are removed.

Paper packets, envelopes, and bags are most commonly used for specimen collection, because they do not gather evidence-destroying moisture or condensation. Nonporous, leakproof, and unbreakable containers are used for collecting and moving liquids, and clean, airtight metal canisters are used to transport arson evidence. Plastic bags are sometimes used to collect dry or powdered evidence. Blood and other moist evidence can be moved from the crime scene to the lab in plastic containers only if the transport time is less than two hours, in order to avoid the introduction and proliferation of contamination-causing bacteria. Upon receipt at the processing area, all items of evidence must be cataloged, then removed and allowed to completely air dry. After drying, evidence can be repackaged in paper or other suitable containers as necessary.

When packaging evidence, it is imperative to avoid cross-contamination by separately and securely packaging and sealing different items. At the start of the custody chain, the evidence container must be clearly marked with the initials of the collector, the date and time of acquisition, a detailed description of both the evidence specimen and the location from which it was collected, and the investigating agency's name and case file number.

The chain of custody typically refers to the paper trail, evidence log, or other forms of documentation pertaining to the collection (whether by sampling or legal seizure), custody, control, transfer, analysis, presentation, and final disposition of material and/or electronic evidence.

In order for evidence to be admissible and credible in court, it is essential that the chain of custody remain intact. Every contact with, or movement of, a piece of evidence must be documented in detail in order to verify that it was never unaccounted for or potentially tampered with. A specific, and appropriately credentialed, individual must be assigned physical custody of individual items of evidence. In law enforcement proceedings, this generally means that a detective will have overall responsibility for the integrity of the evidence; he or she will document its receipt and sign it over to an evidence clerk who is responsible for storing the evidence in a locked and secured area. Every single transaction involving any piece of evidence must be chronologically documented in minute detail from the moment of collection through presentation in court, in order to establish authenticity, and to defend against allegations of tampering. The documentation must include a detailed description of the location and conditions under which the evidence was collected, the identity (and possibly the credentials) of every handler of the evidence, the duration of each movement of the evidence, the level of security for each movement, as well as the overall storage of the item, and a specific description of the manner and conditions under which each transfer of the evidence occurred. If the chain of custody is broken at any time, the evidence is likely to be inadmissible or of minimal, if any, legal value.