Chain of Custody is is a legal definition that many people outside the law don't understand. I see it commonly misrepresented in print and on the big screen so I thought it might be helpful to explain it (at least how it is understood in the United States). As an author you might think that this topic is too wonky. Most readers won't know the difference so why venture into the tall grass right? There's a ring of truth to that line of thinking but, understanding the chain of custody can help you drive a story by introducing conflict. Breaks in the chain of custody can do the same thing in real life!
Basically, the chain of custody is a record of who had possession of each item of evidence at any given time. Notice I say possession and not just proximity, that is a legal difference. More on that in a bit. It is a mechanism to demonstrate that the evidence has been protected and further, who had possession of it so they can be interviewed about their actions if need be. The chain begins the moment the evidence is collected until it is presented in court before the jury. Between those two events the custodian (the person responsible for the evidence) must demonstrate how the integrity was maintained. Law enforcement agencies do this primarily through two mechanisms.
The first is maintaining a written log. The log begins with the first person to collect the evidence from the crime scene, victim, suspect, etc. The form will describe the item (in detail) and have spaces for the names, dates, and times, the evidence changes hands. So as a CSI I may be the first to sign the form. When I "book" the item into the property bureau then the technician or clerk receiving the item will sign for it and I will sign it in the "released by" space. So when reviewing the form you'd be able to determine each and every person that took possession of the item and how long they had possession of it. These forms can list a number of items (say up to twenty) from the same crime scene but each item will have a unique number. Modern agencies use bar code systems but there may still be a physical signature or digital (coded) signature.
The second mechanism is the evidence packaging. When an item is packaged the custodian will seal it with either evidence tape (paper bags/envelopes) or a heat seal (plastic bags). The custodian will then sign their name, employee number, and date across the seal. That way if a lawyer questions whether or not the item could have been accessed by someone not on the form all you have to do is examine the seals. When I testify in court and introduce evidence is it common for me to have to examine a sealed bag and verify my signature and date before opening it for the jury. If I have to examine an item that has been sealed by another person then I always make a new opening (cut) away from the original seal. That way I don't damage the original.
This brings me to an important distinction. Oftentimes I have seen writers describe the chain of custody as "broken" simply because someone came into contact or proximity of the evidence (even when sealed in bags), Usually this is not the case. Consider evidence sent to the FBI crime lab in Quantico, Virginia. Most agencies have to mail their evidence to the crime lab. There is no legal requirement to have each and every mail carrier handling the box to sign a chain of custody form. Tow truck drivers don't sign for vehicles. Likewise, every detective or CSI that enters the property warehouse doesn't have to sign for each item in there just because they came into the vicinity of it. It is only required when you are in a physical position to alter or damage the evidence through physical contact.
So why do I care about this Tom? Boooring! I agree that 99% of the time chain of custody issues are tedious and mundane. But, those moments when the chain is broken can lead to massive fireworks. A compromised chain may force a judge to exclude certain evidence. If that evidence is the murder weapon or a surveillance tape then the whole case may go up in smoke! These mistakes most commonly occur with outside experts unfamiliar with the chain of custody issues. Take a university professor for example. Maybe they are the leading expert on aquatic snails (glamorous I know), the same type you found on your murder victim. This guy may have never worked on a forensic case in his life (or kissed a girl) so the whole experience can be quite exciting. He may not realize that leaving your evidence in his lab where all his grad students and anyone capable of turning a doorknob could gain access to the evidence. Hell, he may even show it off.
These breaks are even more devastating when they are "discovered" during the course of testimony. Imagine the look on the prosecutor's face when they learn their key evidence was compromised! The break in the chain may be unintentional or an act of sabotage. Other occupations that may contribute to a break in the chain could include private investigators, defense experts, or tow truck drivers. I remember a case (not mine thankfully) where an agency towed a suspect vehicle to a storage lot intending to search it the next day. They locked the vehicle but the suspect had a spare key. That night he scaled the fenced lot and stole his own vehicle back! As writers, you can get as creative as you want. A break in the chain of custody can alter the course of a story in much the same way it can alter a case. Evidence can become useless and investigators may have to go back to the drawing board. This all adds up to a ton of conflict and conflict keeps the reader engaged and itching to turn the page. Tempers flare, careers are lost, bad guys get away with crimes, cats and dogs cohabitate; it's a real mess! So get creative and most of all, have fun with it!