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Crime Scene and DNA Basics for Forensic Analysts

Preservation of Evidence

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From crime scene to forensic laboratory to courtroom, all evidence must be inventoried and secured to preserve its integrity. Evidence admissibility in court is predicated upon an unbroken chain of custody. It is important to demonstrate that the evidence introduced at trial is the same evidence collected at the crime scene, and that access was controlled and documented.

An understanding of the rules governing chain-of-custody is vital for an investigator. For example, in a sexual assault incident, the victim is typically transported to another location to have a sexual assault examination performed. Many jurisdictions have established teams to perform these examinations, and they go by several names, such as: Sexual Assault Victim Examination (S.A.V.E.), Sexual Assault Nurse Examiners (S.A.N.E), Sexual Assault Response Team (S.A.R.T) . The examination involves the collection of the victim's clothing, hair samples, swabs for body fluids, and documentation of bruising and bitemarks. The materials collected are packaged by the team members.

Proper evidence packaging includes:

  • Appropriate packaging and labeling of all items
  • Each item properly sealed and marked
  • Correct and consistent information recorded on label and procedural documentation

The evidence is turned over to the investigator for submission to a department's property and evidence section. A receipt documenting the transfer is obtained. Generally, submissions to the forensic laboratory are done on a request for analysis form, listing the evidence items, and a documented chain of custody. Each individual assuming custody of the evidence from collection through analysis signs the chain of custody document. Many departments have automated this process using an information management system, whereby all transfers are securely done using barcodes. The chain of custody report will identify each individual contributing to the analysis of the evidentiary materials.

Once the analysis is complete, the evidence is either returned to the submitting agency or stored by the laboratory. The chain of custody will document this disposition. All law enforcement reports, photographs, lab analysis reports, and chain of custody documents are kept in the case file, which can be made available to the prosecution and is subject to discovery by defense counsel.

Think of the chain of custody as a chain, if one link should be broken, the chain is broken, and the evidence collected may be ruled as inadmissible.

Image of links in a chain
National Institute of Justice (NIJ) (see reuse policy).

Helpful Hints to Safeguard the Chain of Custody:

  • Limit the number of individuals handling evidence.
  • Confirm that all names, identification numbers, and dates are listed on the chain of custody documents.
  • Ensure that all evidence packaging is properly sealed and marked prior to submission.
  • Obtain signed or otherwise secure receipts upon transfer of evidence.

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