Evidence collection is an essential part of a criminal investigation. But there are many chances for evidence corruption, which you could use in your favor at trial.
To avoid such claims, investigators keep a record called the chain of custody. The National Institute of Standards and Technology explains the chain of custody documents the evidence from the point of collection.
The chain of custody begins with the collection at the crime scene. Investigators will carefully document who, how and when they collected the evidence. Then, anytime it changes hands, new documentation is taken. For example, the original person collecting the evidence documents how and when he or she collected it and notes his or her name. When he or she hands that evidence off to someone who will transport the evidence from the crime scene, that person must document when they received it, from who they got it, and any other details. This occurs every single time someone new has the evidence in their possession.
The chain of custody helps ensure that there is always a person who can say they have possession of the evidence. It helps to avoid lost items and reduces the chances of corruption because each person has to verify their possession. In court, each person in the chain of custody could have to give testimony that he or she had possession and what happened with the evidence when they had it and when they passed it on.
A breach in the chain of custody could easily be something you can use in your defense as it opens up questions about the validity and reliability of the item.