What is the Chain of Custody? - Definition, Procedures & Importance

Lesson Transcript
Instructor: Rachel Leintz

Rachel has taught in the fields of Forensic Science and Criminal Justice for over 7 years and has a master's degree in Forensic Science

The chain of custody is used in maintaining criminal evidence throughout an investigation. Look into the definition, procedures, and importance of this process, including documentation, contamination prevention, evidence seals, and tracking. Updated: 11/15/2021

Security and Trust

Imagine you are a crime scene specialist investigating a homicide. During the investigation, you find a drop of blood on the floor, near the doorway, leading away from the victim's body. This could definitely be important evidence, so it needs to be collected.

How do you go about doing this? If the blood ends up being critical to the case, you will have to testify in court that you collected it. You'll have to explain where you found it and how it was collected, and you will have to swear, under oath, that the swab presented to the court is the same one you collected. The trial might not take place for months or even years! How will you be sure this evidence is tracked, logged, and protected from tampering?

The processes for documenting, collecting, and protecting evidence are called establishing a chain of custody. Creating and maintaining a chain of custody means that a detailed log is kept of where the evidence was found and logging anything that happens to the evidence prior to trial.

For this lesson, let's follow the documentation, collection, and packaging process for the blood on our homicide investigation. You'll see all the different steps required for creating and maintaining a chain of custody when it comes to crime scene evidence.

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  • 0:00 Security And Trust
  • 1:16 Documentation,…
  • 2:34 Preventing Contamination
  • 3:18 Evidence Seals And Tracking
  • 4:35 Lesson Summary
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Documentation, Documentation, Documentation!

Chain of custody always begins with crime scene specialists, or the professionals conducting the crime scene investigation. When evidence is first found on a crime scene, steps have to be taken to document where that evidence was found. This is usually done with crime scene photographs and notes taken during the initial investigation.

Most police agencies and crime labs will label items of evidence with a number once it has been located. In the picture below, the investigator has labeled the crowbar as item #1 and is photographing it in place. This is usually the starting place for documenting the location of the evidence.

Crime scene photographer documenting evidence
crime scene photographer

Once the item has been photographed, crime scene investigators will take measurements of the evidence to further document its location. There are several methods for measuring evidence on a crime scene, but one of the most common methods is the rectangular coordinates method.

To use this method, the investigator picks a fixed point in the scene (or a reference point), such as the northwest corner of the master bedroom, then measures the distance to the evidence from that point. Let's use our blood as an example. This simple sketch shows us that the blood was found on the floor, eight feet and nine inches east of the fixed point and nine feet and two inches south of the fixed point:

Example evidence sketch using the rectangular coordinates method
crime scene diagram

Preventing Contamination

After the blood stain is documented in place with photographs, notes, and sketches, we are now ready to collect it. In the example, you will need to collect the blood using a sterile swab. Because you are touching the swab to the evidence (the blood), it needs to be sterile, so that you can be sure you aren't contaminating the evidence with anything that might have been on the swab. You will also need to wear gloves while doing this, so that you don't contaminate the swab with any of your own DNA. Wearing gloves is a cardinal rule for almost all crime scene investigations. If the investigation requires it, additional PPE, or personal protective equipment, like shoe covers, hair nets, masks, goggles, and body suits, may be required.

People wearing personal protective equipment

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