When investigators collect evidence at a crime scene, they must follow certain procedures. One of the most important things they must do is to follow the chain of custody procedures.
The National Center for Biotechnology Information defines the chain of custody as the process of documenting evidence that accounts for its possession and location from collection to presentation in court. The process is essential to preserving the integrity of the evidence. It helps to protect against contamination or other alteration of the evidence that could result in the court throwing it out at trial.
The chain of custody begins at the crime scene when investigators collect the evidence. At this point, whoever collects the evidence must then properly document it and ensure it remains in protective custody until it goes to the laboratory for examination. At the laboratory, the investigator transfers custody of the evidence to the person at the lab who will examine it.
After the lab, the chain of custody often goes back to law enforcement who stores the evidence for the prosecutor. When the time comes, the prosecutor takes custody of the evidence to introduce it to the court.
There must be an account of every person who has control over the evidence and every place it has been since its collection from the crime scene. Any missing information could introduce doubt as to the authenticity of the evidence. The defense could then ask to exclude the evidence. This could hurt the prosecution’s case.
For the defense, the chain of custody helps to ensure any evidence the prosecutor introduces is true and accurate evidence. It helps to prevent tampering and the potential for false evidence introduction in a trial. The chain of custody also ensures there are no rights violations and that everything involved in the collection and handling of the evidence is lawful. In short, it is one of the things done in a criminal case to ensure the rights of the accused to a fair trial.