Chain of Custody in Offences Against Public Health: Legal Nature, Effects of a Break and Strategy for Challenging It

In proceedings for an offence against public health (delito contra la salud pública, the heading under which Spanish law punishes drug trafficking: arts. 368 et seq. of the Spanish Criminal Code, or CP), proof of the nature, weight and purity of the substance rests, as a general rule, on expert evidence carried out on a sample. The probative value of that expert report depends on what was analysed being what was seized. That is why challenging the chain of custody is a recurring line of defence and, at the same time, one of the least likely to succeed.

This article sets out systematically the case law of the Tribunal Supremo (Spanish Supreme Court, whose judgments are cited as “STS”) and of the Audiencia Provincial de Málaga (Málaga Provincial Court, cited as “SAP Málaga”) on the issue, quoting the decisions verbatim.

1. Concept and rationale: the guarantee of “sameness” (mismidad)

The chain of custody (cadena de custodia) refers to the set of acts by which the evidence obtained in a criminal investigation is collected, transferred and preserved, acts which must meet requirements designed to ensure the authenticity, unalterability and integrity of the sources of evidence (STS 1939/2020 of 31 March; SAP Málaga 4536/2025 attributes this definition to STS 147/2015).

Its rationale is to guarantee the identity of the object of evidence. STS 3116/2012 of 24 April puts it as follows:

“is to guarantee that, from the time the traces connected with the offence are collected until they take concrete form as evidence at trial, what will be subject to the immediacy, publicity and adversarial testing of the parties and to the judgment of the court is one and the same thing.”

The same judgment identifies three stages subject to scrutiny: “What is found must be described and taken with due safeguards, placed in storage with due safeguards, and analysed with due safeguards”. As its statutory reference point it cites Article 318 of the Spanish Criminal Procedure Act (Ley de Enjuiciamiento Criminal, hereinafter LECrim), which provides for sealing, retention, preservation or dispatch to the appropriate body, together with the duties of documentation and of reporting to the judge under Articles 292 and 295 LECrim.

A point of context: STS 3116/2012 itself observes that the regulation of the custody procedure has largely been left “to the good practice of those involved in the criminal investigation”. The absence of a detailed regime explains why scrutiny is carried out case by case and through the assessment of the evidence.

2. Nature: instrumental value and relationship with expert evidence

The Tribunal Supremo has repeatedly held that the chain of custody is not evidence in itself. SAP Málaga (Section 3) 4536/2025 of 1 October reproduces the doctrine of STS 795/2014, which in turn cites STS 1/2014:

“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 time it is collected until it is analysed, so that a break in it may affect the credibility of the analysis, but not its validity.”

Its connection with expert evidence is direct. STS 1939/2020 records that “the validity of the results of the expert examination depends on the guarantee as to the origin and content of what is being analysed” (STS 587/2014). For that reason, it adds, an infringement of the chain “affects what is known as the verisimilitude of the expert evidence and, consequently, its legitimacy and validity to serve as incriminating evidence (prueba de cargo)”.

There is a nuance regarding the nature of the initial police action. According to STS 3116/2012, collection and custody do not amount to pre-constituted expert evidence but to “a police investigative step that acquires evidential relevance, as witness evidence, when the officers appear at the oral trial to testify about what they perceived through their senses”. This places the centre of gravity at trial rather than in the police report (atestado).

3. Irregularity and break: two different planes

Case law distinguishes between two planes that the defence must not confuse.

a) The plane of lawfulness (validity). This is the plane of unlawful evidence (prueba ilícita), obtained in violation of fundamental rights (art. 11.1 of Spain’s Organic Law on the Judiciary, LOPJ). Case law holds that the chain of custody does not belong to this plane. STS 4652/2025 of 22 October puts it this way:

“It is not a precondition of validity but of reliability. When the chain of custody is broken we do not enter the field of the unlawfulness or unusability of evidence, but that of reduced reliability (impaired or even destroyed) because certain safeguards have not been respected. They are two different planes. Unlawfulness cannot be cured.”

The contrast with unlawful evidence has consequences for the applicable regime. Unlawfulness cannot be cured and may extend its effects to derivative evidence, unless the causal link or the link of unlawfulness (desconexión de antijuridicidad) is severed. A break in the chain, by contrast, bears on the weight of the evidence and can be offset by other evidence.

b) The plane of reliability (assessment of evidence). This is where the chain of custody belongs. STS 3116/2012 explains that a failure to observe formalities constitutes a procedural infringement which does not render the investigative step null, although it makes the evidence irregular, and that:

“the loss of guaranteed authenticity caused by the formal deficiency could go as far as the complete disqualification of the expert evidence only if the chain of custody offers no guarantee whatsoever”

Of the formulations analysed, this is the one most favourable to the defence: complete disqualification exists as a possibility, but it is reserved for cases in which there are no safeguards at all.

c) Distinguishing a mere irregularity from a relevant break. STS 3699/2015 of 27 July sums it up: “an irregularity in the protocols established to safeguard the chain of custody is not equivalent to nullity”. What must be determined is whether the irregularity (failure to record a mandatory item of information, lack of precise documentation of a step) “is capable of raising doubts as to the authenticity or integrity of the source of evidence”.

STS 286/2023 of 12 January listed the deficiencies it regards as “mere irregularities or formal defects”:

“1.- Defective or erroneous numbering of the boxes containing the source of evidence. 2.- The case reference number is not recorded. 3.- There is no record of the dispatch of the physical items from the time they were collected until their delivery to police premises. 4.- Lack 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.”

It concludes that these and similar cases “do not sow doubts as to the identity of the substances or objects seized”, since they correspond to what was seized by the police.

4. Presumption of regularity and burden of allegation

Case law operates with a rebuttable presumption (iuris tantum) that what was collected corresponds to what is brought before the trial court:

“there is a presumption that what was gathered by the judge, the expert or the police corresponds to what is presented as evidence on the day of the trial, unless there is a reasonable suspicion that some kind of possible tampering has occurred.” (STS 3116/2012)

STS 1939/2020 reinforces that presumption with regard to initial police custody, pointing to “the impossibility of accepting that police actions are illegitimate or irregular, as long as the contrary is not established or cannot be inferred”.

This presumption gives rise to a heightened burden of allegation for the party bringing the challenge. SAP Málaga (Section 2) 563/2024 of 3 April invokes STS 990/2016 and specifies that “raising doubts of a generic nature” is not enough; there must be a reasonable doubt. The party must specify “at what points, as a result of what actions and to what extent such an interruption has occurred, and may propose at first instance the evidence aimed at proving it”.

There is also a timeliness requirement. SAP Málaga 4536/2025 pointed out that the issue, raised only in the closing argument, “could well have been raised at the preliminary-issues stage” (cuestiones previas), which signals the risk of the objection being held to be out of time.

5. Standard of assessment: the reasonable-doubt threshold

For a break to have any effect, case law requires that it give rise to a rational doubt as to the sameness of the item analysed. STS 1709/2023 of 19 April adopts the doctrine of STS 129/2015:

“Only if the formal deficiencies give rise to serious rational doubts should this source of evidence be disregarded, not because of a failure to comply with some step or procedure laid down in the protocol for the receipt and custody of samples, but because its authenticity has been called into question.”

The methodology that emerges is that of an overall assessment of the evidence. SAP Málaga 4536/2025, citing STS 777/2013, puts it in these terms: the absence of statutory safeguards leads “to a comparison of all the evidence in order to determine whether evidential doubts have arisen, which must always be resolved in favour of the defendant; but not to the discrediting of that evidence without further inquiry”.

Two examples of how the standard is applied:

• In STS 1709/2023 the complaint that the substances had not been placed at the disposal of the court, that not all the officers had been identified and that the place of storage was unknown was rejected. The court took into account that the handover record and the expert report matched as to the file number, the police report and the investigating court, which “shows that the substance seized is the same as the one delivered and analysed”.
• In STS 4286/2024 of 11 July, the Supreme Court emphasised that the officials involved in the receipt and analysis of the substance appeared at trial, “the defence being able to put to them all the questions it considered relevant regarding the chain of custody and the manner in which the analyses were carried out”.

6. Procedural route and review in cassation

A break in the chain of custody has no ground of cassation appeal of its own. In the decisions analysed, it is channelled through three routes.

1. Violation of fundamental rights (arts. 852 LECrim and 5.4 LOPJ). The right to proceedings with all safeguards and the right to the presumption of innocence (art. 24.2 of the Spanish Constitution, CE) are invoked. This is the most common route (STS 4652/2025, STS 3699/2015, STS 1709/2023). SAP Málaga 4536/2025, citing STS 545/2012, specifies that the infringement may be significant in cassation “not as a mere finding of the alleged infringement of administrative rules, but because of its hypothetical impact on the right to the presumption of innocence”.
2. Breach of procedural form owing to the refusal of evidence or a failure to respond (arts. 850.1 and 851 LECrim). This route was used in STS 1709/2023 to argue that the defence’s request concerning the chain of custody had not been ruled upon. The Court’s answer focuses on whether there is a relevant irregularity giving rise to doubt about the transfer of what was seized.
3. Infringement of law owing to failure to prove the object of the offence (art. 849.1 LECrim). In STS 1709/2023 it was argued that the conduct did not fall within the offences defined in arts. 368, 369 and 370 CP, because the existence of a narcotic substance had not been proven, in connection with the chain of custody.

It should be borne in mind that this ground operates within the review of the assessment of evidence. STS 286/2023 holds that a complaint of a break in the chain “lacks constitutional scope” and that it is “an issue affecting the assessment of the evidence, but not its validity”. The Supreme Court usually confines itself to checking that the trial court has given logical reasons for dismissing the doubt. Where it has done so, the ground of appeal is dismissed.

7. Complex cases: controlled-delivery operations and extended chains

In operations involving foreign authorities, the chain is long and is documented at source. In STS 4652/2025 the defence alleged gaps between the handover of the packages in Caribbean waters (25 September 2022) and their dispatch to Madrid (8 October), owing to the absence of any record of unloading, of transfer to the airport or of the identification of the officers holding custody.

The Court dismissed the ground. It found that the trial court had analysed “in detail all the evidence taken at the oral trial, both witness and documentary” and that the experts’ analysis, confirmed at trial, established that the substance analysed was the one held in custody by the police officers. The judgment is useful as a benchmark for gauging the level of proof required: the documentation from the country of origin is combined with the witness evidence given at trial.

8. Practical implications for the defence

The following guidelines can be drawn from the case law analysed:

1. Pinpoint the specific link in the chain being challenged (seizure, storage, transfer, receipt at the laboratory, sampling, destruction) and the circumstance giving rise to the doubt.
2. Cross-check the documentation (seizure record, handover record, receipt record, expert report) as regards dates, weights, number of packages, description and identification details, as was done in STS 1709/2023.
3. Propose evidence to prove the break at first instance, in line with the requirement laid down in STS 990/2016, rather than holding the issue back for the closing argument.
4. Examine at trial the seizing officers, the custodians and the experts on the specific aspects of custody (STS 4286/2024).
5. Review the applicable weighing and sampling rules. Some appeals question compliance with the Framework Agreement (Acuerdo Marco) of 3 October 2012 and its Practical Action Guide (Guía Práctica de Actuación); see STS 286/2023, in relation to how the parts of the cannabis plant are counted. Quantity and purity determine whether the conduct falls under the basic offence or under the aggravated sub-types.
6. Frame the ground in terms of the presumption of innocence, so that what is challenged is the insufficiency of the incriminating evidence owing to a reasonable doubt as to sameness, and not a mere breach of protocol.

Conclusion

According to the case law analysed, the chain of custody operates as a guarantee of the authenticity of the expert evidence and not as a requirement for its validity. A break in it does not, on its own, lead to the exclusion of the evidence, unless the chain offers no guarantee at all, and it is resolved at the level of the assessment of evidence under the reasonable-doubt standard. A defence seeking to draw consequences from it must identify the link affected, prove its impact on the identity of what was analysed, and raise the issue in good time and with supporting evidence. Without that precision, the courts apply the presumption of regularity and dismiss the ground.

For the assessment of circumstantial evidence and the most common nullity claims in these proceedings, see our article on circumstantial evidence and nullity claims in offences against public health.

This article is for information purposes only and is no substitute for legal advice on a specific case.

Are you facing proceedings for an offence against public health in which the expert evidence or the chain of custody is in dispute? Contact the firm for an initial confidential assessment of your case.

Dr. Álvaro Heredia Lara. Criminal lawyer, member no. 10.846 of the Ilustre Colegio de Abogados de Málaga (Málaga Bar Association).