The chain of custody: the guarantee that evidence is authentic
Imagine the police find a DNA sample of the suspect at the crime scene, but on arrival at the laboratory the packaging is open and there is no record of who collected it or how it was transported. Does that sample have evidentiary value? The answer of modern criminal proceedings is: it depends, and in many cases no.
The chain of custody is the set of procedures that guarantee that an item of evidence is the same one collected at the scene and that it has not been altered, contaminated, substituted or manipulated between its obtaining and its presentation at trial. It is, in essence, the guarantee of the integrity of physical and digital evidence. It is not a bureaucratic formality: it is a substantial guarantee of the right to effective judicial protection and of the presumption of innocence, because evidence whose chain of custody cannot be established cannot be assessed with the certainty necessary to convict.
A correct chain of custody must document several elements. The initial collection: who collected the evidence, when, where, in what condition and with what preservation measures; errors at this moment, such as collecting DNA without gloves or handling a weapon without protection, are difficult to remedy. The transport: each transfer must be recorded, with its conditions, because biological samples can degrade at inappropriate temperatures. The storage: in conditions that prevent degradation and with restricted and recorded access. The analyses: each expert manipulation must be documented, indicating methodology, results and quantity of sample consumed. And the presentation at trial, where it must be verified that what is presented is what was collected.
Digital evidence poses particular challenges: files are very sensitive to modifications, and any access can alter their metadata. The standard is the use of cryptographic hash functions, which produce a digital fingerprint of the file that changes in a detectable way if the content is modified. In addition, analyses must be carried out on forensic images, that is, exact copies, and not on the original.
The breaking of the chain of custody has consequences, although not automatic: case law requires that the irregularity has generated a reasonable and real doubt, not merely theoretical, about the integrity of the evidence. If that doubt exists, the consequences may be the exclusion of the evidence, the reduction of its evidentiary value or the application of in dubio pro reo.
For the defence, the chain of custody is a valuable and under-explored tool. The lawyer must request access to all the custody documentation of each item of evidence, analyse it with the help of an expert who detects technical irregularities and articulate those irregularities as grounds for challenge in a technically solid way, identifying which specific link fails.
The particularities vary according to the type of evidence: in biological samples the risk is contamination and degradation; in narcotic substances, establishing that the one analysed is the one seized; in weapons and objects, preserving fingerprints and traces; and in digital evidence, the technical integrity mechanisms. In Spain there is no single rule on the chain of custody, but rather a regulation dispersed among the Criminal Procedure Act, forensic protocols and the case law of the Supreme Court.
