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The term 'chain of custody' refers to the protocol for handling physical proof that will be introduced in a courtroom, ensuring evidence complies with the rules of criminal procedure. Basically, there are rules for managing different types of evidence and keeping it reliable, and this is the set that applies to the admissibility of material things.
Cases are proven using evidence, and chain of custody dictates how the proofs are collected, stored, and displayed in court. Evidence that has not been documented properly and preserved according to the rules can be excluded from a case. So, in a murder say, if the cops find the silver bullet but handle the evidence it all wrong, the defense can file a motion and that proof is gone.
Why We Need the Chain
Mike Byrd, a former Miami-Dade Police Crime Scene Investigator wrote extensively about chain of custody. For investigators, proper chain of custody begins with careful tagging and labeling of evidence.
He explained, "The chain of custody is ... the witnessed, written record of all of the individuals who maintained unbroken control over the items of evidence. It establishes the proof that the ... evidence collected at the crime scene is the same evidence that is being presented in a court of law."
Specifically, chain of custody establishes the following:
Establishing Custody in Court
Prosecutors have to bring in witnesses who handled the evidence and establish where it was found, how it was handled, who handled it, and what -- if anything -- has happened to the evidence since it was found. When the chain of custody is broken or suspect, it makes the evidence untrustworthy.
A defense attorney can either file a motion to dismiss evidence based on a faulty chain, or try to create doubt in a jury during trial by cross-examining witnesses about their handling of evidence. If jurors suspect that the evidence was mismanaged they may not convict, which is correct according to American legal standards.
Beyond a Reasonable Doubt
The legal standard for a criminal conviction in the U.S. is proof beyond a reasonable doubt. That means that any evidence on which a conviction is based must be absolutely reliable. If there is a question about the handling of evidence by authorities, that is enough to constitute reasonable doubt.
Prosecutors must provide reliable evidence because juries have an enormous responsibility. They decide when a crime has been proven and who is guilty. Chain of custody is just one of the ways that the legal system tries to ensure that authorities do not abuse power and that justice is served.