In criminal cases, “chain of custody” is an evidentiary term used to describe the tracking of real/physical evidence (often contraband or weapons) from the time it was taken into custody by police until the prosecution attempts to introduce it at trial.
As such, it is an evidentiary burden that the government must satisfy to assure the judge/jury that the evidence is what the prosecution claims it to be (that is, that it has not been contaminated or tampered with).
Chain of custody is usually an issue in cases in which the evidence is fungible. In a drug case, for example, the government will typically introduce testimony that the substance was confiscated from the defendant’s person, put into a heat-sealed envelope that is marked for identification, taken to the laboratory for testing, tested by an analyst to determine its composition, put back into another heat-sealed envelope and again marked for identification, and then stored in a secure place until trial.
The government might not need to prove chain of custody in cases in which item is easily identified through distinct markings. In that case, it may be sufficient for a witness to identify the item based on these unique characteristics.
In cases in which there is a break in the chain of custody, the court may preclude the evidence from being admitted. At the same time, “reasonable probability” that the evidence was handled correctly is not a rigid standard and the court has considerable discretion in deciding whether or not to admit evidence.
As spelled out in greater detail in Plummer v. United States, 43 A.3d 260, 272 (D.C. 2012), the government also has the benefit of an evidentiary presumption that physical evidence that is in the hands of the government is handled properly. Id.
Once the government has established an unbroken chain of custody as a matter of reasonable probability, the burden shifts to the defense to show that the police tampered with or otherwise mishandled the evidence.
Once the evidence has been admitted, chain of custody issues may be used to challenge the weight of evidence that is introduced against the defendant.