Physical evidence that is received, recovered, discovered or collected in any manner by an investigator must be marked for identification without delay. The purpose of identification is to prove at a later date, such as during a trial, that the evidence presented is the same evidence that was originally collected. Initials, time, date and place of collection are the usual forms of identification.

Depending on the size and nature of the evidence, the identifying information can be attached to the item or on the outside of a container suited to the evidence. For example, a crow bar can be appended with a tie-on tag, a paint chip can be placed in a plastic sandwich bag, clothing can be placed in a paper bag, a fluid can be placed in a vial and dirt or debris can be placed in a metal can. The central concept is to prevent contamination of the evidence through contact with a contaminating substance, prevent accidental or willful substitution, and positively establish the source of the evidence and identify persons or agencies that had custody of the evidence from the moment of collection to final disposition. Every person in the chain of custody is responsible during the period of custody to provide care and safekeeping.

Source: Learning Shop USA, a provider of continuing education for private investigators.