How do we ensure the identity between the facts, the police intervention and the court's knowledge of them?
What is the chain of custody and why does it exist?
When the police seize a substance, an object or any trace connected to an offence, that item does not reach the courtroom directly. It must first be collected, described, packaged, transported, deposited and, very often, analysed by a laboratory or an expert. Along that route, which may take months and involve several different people and bodies, there is always at least a theoretical risk that what is ultimately presented as evidence at trial is not exactly the same thing that was collected at the scene, or that it has been altered, contaminated or mixed up with other evidence.
The chain of custody is precisely the set of guarantees and protocols designed to prevent that risk. The Supreme Court summarises the problem the chain of custody solves as follows:
"[The purpose] is to guarantee that, from the moment the traces connected to the offence are collected until they come to constitute evidence at the time of trial, that which will be subject to the immediacy, publicity and adversarial examination of the parties and the court's assessment is one and the same thing. It is through the chain of custody that the 'sameness' of the evidence is satisfied, since the seized substance must pass through different places for the various examinations to be carried out, it is necessary to be certain that what is transported and analysed is the same thing at all times, from the moment it is seized until the final moment when it is studied and analysed and, where applicable, destroyed." (STS 375/2021, of 5 May).
From this arises the so-called principle of "sameness", that is, the guarantee that the item seized, the item analysed and the item tried are one single and identical evidentiary reality. This is not a decorative requirement: if sameness cannot be guaranteed, the very factual basis on which the prosecution rests is called into question, because the court would be assessing something different from what actually happened at the scene of the offence.
This same idea appears formulated in almost identical terms in a terrorism case decided years earlier, in which the defence challenged DNA evidence obtained from a coffee cup at the crime scene:
"We have said that it is through the chain of custody that the guarantee of what has been called 'the sameness of the evidence' is satisfied (STS 1190/2009, of 3 December). [...] the chain of custody is a concept taken from reality and given legal significance, for the purpose of fully identifying the object seized, since, as it must pass through different places for the relevant examinations to be carried out, it is necessary to be certain that what is transported and analysed is the same thing at all times, from the moment it is collected at the scene of the offence until the final moment when it is studied and, where applicable, destroyed." (STS 607/2012, of 9 July).
To give effect to this guarantee, the law imposes specific obligations at each stage of the process. Article 318 of the LECrim (Spanish Criminal Procedure Act) requires that the instruments, weapons and effects of the offence be sealed where possible, and that their retention, preservation or transfer to the appropriate body for deposit be ordered; and, in the case of narcotic drugs, Article 3 of Law 17/1967 of 8 April requires their delivery to the Narcotics Control Service for analysis in accordance with regulated scientific protocols. It is on this statutory basis, collection, sealing, deposit and analysis carried out with guarantees, that the entire body of case law set out below is built.
An instrumental guarantee, not a stand-alone fundamental right
The second pillar of this line of case law, and perhaps the one that causes the most confusion in practice, is that the chain of custody does not, in itself, constitute either evidence or a fundamental right. It is an instrument that serves the reliability of the expert or material result presented as evidence for the prosecution.
The Supreme Court explains this, citing its own earlier case law:
"An irregularity in the 'chain of custody', if that is the case, does not in itself amount to a violation of any fundamental right, which would only arise from admitting and giving weight to evidence obtained without respecting the essential guarantees of the proceedings and, in particular, the right of defence; and, secondly, that the formalities which must be observed in the seizure, preservation, handling, transport and delivery to the destination laboratory of the substance to be examined, the process generically referred to as the 'chain of custody', has a merely instrumental character, that is, it serves only to guarantee that the substance analysed is the same and complete substance seized, generally, at the outset of the proceedings." (STS 375/2021, of 5 May).
The idea of the chain of custody as an instrumental guarantee, rather than a fundamental right, is also the starting point of STS 332/2019, which formulates it by citing another judgment of the same Chamber:
"in the words of STS 1/2014 of 21 January, the chain of custody is not an end in itself, but has an instrumental value. All it guarantees is the integrity of the evidence from the moment it is collected until it is analysed, which, if broken, may affect the credibility of the analysis but not its validity." (STS 332/2019, of 27 June, citing STS 541/2018, of 8 November).
And it adds, on the same point:
"the chain of custody is not evidence in itself, but serves as a formal guarantee of the authenticity and integrity of the expert evidence. Its breach affects what is termed the verisimilitude of the expert evidence and, consequently, its legitimacy and validity to serve as evidence for the prosecution in the proceedings." (STS 332/2019, of 27 June, citing STS 587/2014, of 18 July).
This distinction, between the chain of custody as an instrumental guarantee and the fundamental rights that a break in it may genuinely compromise (the right of defence, the right to due process with all guarantees, the presumption of innocence), is the key to understanding why not every break in the chain produces the same legal effect. It is also the gateway to the question that matters most in practice: when does a break render the evidence void, and when does it merely weaken it?
Breaking the chain of custody: why not every irregularity amounts to nullity
The Supreme Court's consistent answer is that a break in the chain of custody does not automatically amount to nullity of the evidence. This is stated with particular clarity in STS 195/2014, deciding an appeal in which discrepancies had been detected between different weighings of a narcotic substance carried out over the course of the proceedings:
"An irregularity in the protocols established as a guarantee for the chain of custody does not amount to nullity. It must be assessed whether that irregularity (failure to record any of the data required to be entered; absence of exact documentation of any of the steps...) is capable of raising doubts about the authenticity or integrity of the source of evidence. It is not a matter of nullity or unusability, but of reliability." (STS 195/2014, of 3 March).
This sentence sums up the central criterion: nullity and reliability are different planes. An irregularity may weaken the evidentiary value of an analysis, and be taken into account by the court when assessing whether the evidence is convincing, without this meaning it must be excluded from the proceedings. In the case decided by this judgment, the appellant argued that the weight discrepancies should lead, "at most", to a conviction under the basic offence of Article 368 of the Criminal Code instead of the aggravated subtype applied; the Supreme Court rejected that consequence, noting that the accused had had the opportunity, both during the investigation and at trial, to request further analyses and to question the experts and officers involved, so that no defencelessness had occurred.
STS 332/2019 develops this criterion by offering a non-exhaustive catalogue of defects that the Chamber regards as mere formal or bureaucratic irregularities, insufficient on their own to call into question the "sameness" of the evidence:
"1.- Defective or erroneous numbering of the boxes containing the source of evidence. 2.- The case/proceedings number is missing. 3.- There is no record of the document evidencing transfer of the items from their collection until their delivery at the police station. 4.- Absence of a seal. 5.- Inadequate packaging that does not affect the sample or the information that can be extracted from it; or 6.- Mere delay in sending the seized substance to the laboratory for analysis." (STS 332/2019, of 27 June).
"This Chamber adds that these and other similar cases do not cast doubt on the identity of the substances or objects seized, since they correspond to what was seized by the police. These are disruptions of a rather bureaucratic nature which, in principle, unless accompanied by another series or set of irregularities that jeopardise the security of the chain of custody, need not call into question the authenticity and sameness of the traces and evidence underlying the evidence for the prosecution." (STS 332/2019, of 27 June).
As against these minor irregularities, the same judgment identifies the situations that can genuinely compromise the validity of the evidence:
"And turning now to what can actually amount to an infringement of this chain, it may be said that quite different are cases of serious alteration, contamination, destruction or loss of the samples, effects or instruments used in the criminal act, or even where administrative irregularities give rise to significant legal uncertainty as to the place and persons where the factual items were held." "Only if the formal deficiencies raise serious, rational doubts should this source of evidence be disregarded, not because of failure to comply with some step or procedure set out in the protocol for receiving and safeguarding samples, but because its authenticity is called into question." (STS 332/2019, of 27 June, citing STS 129/2015, of 4 March).
And, to prevent any generic allegation from serving to call the whole of the evidence into question, the Chamber imposes a precise burden of argument on the party invoking the break:
"In order properly to examine whether a material break in the chain of custody has occurred, it is not enough to raise doubts of a generic nature; the party challenging it must specify at what points, as a result of what actions, and to what extent such an interruption occurred." (STS 332/2019, of 27 June).
Ultimately, it is the seriousness of the impairment, not the mere existence of a formal defect, that determines whether we are dealing with a question of reliability (which the court weighs when assessing the evidence) or a question of validity (which may lead to its exclusion).
When nullity does apply: fundamental rights and the "fruit of the poisonous tree" doctrine
All of the above changes when the way in which the evidence was obtained, held or used is not a mere protocol defect but has involved a violation of a fundamental right, in particular, the right of defence or the right to due process with all guarantees under Article 24 of the Spanish Constitution. Here Article 11.1 of the Organic Law on the Judiciary (LOPJ) comes into play, providing that evidence obtained, directly or indirectly, in breach of fundamental rights or freedoms shall have no effect.
It is on this ground, and not on that of mere protocol irregularity, that the well-known "fruit of the poisonous tree" doctrine applies, originating in the case law of the Constitutional Court and adopted by the Supreme Court: if the original evidence was obtained in violation of a fundamental right, that violation may also "contaminate" evidence derived from it, even where the latter was itself obtained in a formally correct manner. Nullity, in such a case, is not confined to the tainted act but extends to everything that flows from it.
This explains why the effect of a break in the chain of custody cannot be analysed in the abstract, but depends on which specific guarantee has been affected:
On the one hand, if what fails is an internal protocol for handling or transport, without this compromising a fundamental right, we are in the realm of reliability; the defect is therefore weighed by the court but does not contaminate the rest of the proceedings. On the other hand, if the manner in which the evidence was obtained violated the right of defence, for example, because a person gave a statement that was later used against them without having had the legal assistance to which they were entitled once the investigation was directed at them, the consequence may be the nullity of that specific piece of evidence, under Article 11.1 LOPJ, and that nullity may in turn drag down evidence obtained exclusively from it, in accordance with the "fruit of the poisonous tree" doctrine.
It should be stressed that this last scenario, a statement given without legal assistance that is later used against the person who gave it, reflects a general doctrinal development on unlawfully obtained evidence and its knock-on effect, rather than a specific pronouncement located in the judgments we have been able to verify for this piece. Before using it in a submission addressed to a specific case, a judgment expressly supporting it should be located and verified.
For this reason, when faced with an allegation that the chain of custody has been broken, the first question defence counsel should ask is not "is there a defect?", but "what guarantee does that defect protect, and is it a matter of mere protocol or a constitutional guarantee?" The answer determines whether the outcome, in the best case for the defence, will simply be a loss of evidentiary weight for an expert report, or the nullity of the evidence and, potentially, of everything that depends on it.
Conclusion
The chain of custody protects the "sameness" of the evidence, but does so as an instrumental guarantee, not as a stand-alone fundamental right. Two practical consequences follow from this, which should always be kept in mind:
- Not every break or irregularity results in nullity of the evidence. Purely formal or bureaucratic irregularities, numbering, seals, delays, minor discrepancies, are assessed as a question of reliability, within the court's free assessment of the evidence.
- Nullity only comes into play when the break is serious enough to raise reasonable doubts about the authenticity of the source of evidence, or when the alleged defect actually amounts to a violation of a fundamental right (Article 11.1 LOPJ). Only in this second scenario does it make sense to invoke the "fruit of the poisonous tree" doctrine to extend nullity to derived evidence.
Correctly distinguishing between these two planes, reliability and nullity, is, in practice, the key to any defence strategy that seeks to successfully challenge a chain of custody.
