1 Introduction

The stockpile, production, development, or use of chemical weapons is comprehensively banned under the 1997 Chemical Weapons Convention (CWC) and customary international law. Given the wide range of legitimate and peaceful uses of chemicals, the CWC’s general purpose criterion defines any toxic chemicals as weapons unless they were developed in types and quantities for purposes not prohibited under the Convention, which are listed in Article II paragraph 9 (OPCW 1993a). To enforce this criterion, the CWC encompasses procedures for its implementing body, the Organization for the Prohibition of Chemical Weapons (OPCW), to verify the peaceful use of chemicals and compliance of state parties with their obligations under the convention and identify cases of alleged development and use of chemical weapons.

While Syria was not a state party to the CWC during the first cases of alleged chemical weapons use in Syria during the civil war, it acceded in 2013 to the Convention. Yet, even after accession, procedures such as challenge inspections into potential non-compliance, as explicitly foreseen in the Convention, were not fully employed. Instead, several ad-hoc mechanisms were established to investigate alleged use or further attribute responsibility based on varying legal frameworks. With distinctive mandates, and depending on the objectives pursued, the mechanisms followed and adopted different working methods and technology in meeting and achieving varying degrees of certainty for the findings—referred to in this paper as Standards of Proof. The paper identifies, compares and contrasts the working methods and standards of proof of the mechanisms and provides explanations for technical and legal differences.

2 Methodology

The paper studies the cases of four investigation mechanisms established and operated in the context of the chemical weapons use in Syria. By reviewing available resources about the mechanisms, including about 40 official reports,Footnote 1 the paper identifies the scientific and technical working methods and their role in meeting the legal objectives and their respective standards of proof.

To that end, the paper attempts to provide insights into the interlinked relationship of technical and legal standards as well as mandate and founding body, and objectives of the mechanism. This is done by analyzing and comparing the number and the nature of the working methods used in each mechanism with the applicable standard of proof. In theory, science and technology improve the quality of working methods and increase the chances of reaching conclusive findings. Therefore, more sophisticated working methods should enable a higher degree of certainty. Yet, comparing the different mechanisms may prove that other external factors create a more substantial influence on the standard of proof than technical and working methods.

3 Chemical weapons investigation mechanisms in Syria

Article IX of the CWC and Part XI of the Verification Annex outline the chemical weapons prohibition regime’s mechanism for fact-finding in cases of possible non-compliance. State parties have the right to request a challenge inspection to investigate allegations in any territory under the jurisdiction of any state party at any time without the right to reject (Krutzsch et al. 2014). To avoid abusing this right, a request may be dismissed in cases determined by the OPCW’s Executive Council if it is found to be unreasonable or out of the CWC’s scope. However, to this day, there is no precedent for invoking this right, not even in the case of Syria (Abe 2017). When the first allegations of chemical weapons use in Syria were reported at the end of 2012, Syria was not yet a party to the CWC. Therefore, the OPCW was not inherently tasked with investigating the incident. However, in such cases involving states not party to the CWC, the OPCW can cooperate with the UN Secretary-General, putting all resources at their disposal (OPCW 1993b).

Upon several calls from state parties to investigate 16 allegations of chemical weapons use reported in 2013, the United Nations Secretary-General’s Mechanism for Investigation of Alleged Use of Chemical and Biological Weapons (UNSGM) was the first to be deployed to Syria with a mandate limited to confirming whether the use of chemical weapons took place, but without attributing responsibility. The UN led this mission but received technical support from OPCW experts. Of the seven allegations investigated, the UNSGM concluded that chemical weapons were used in five cases but couldn’t corroborate the allegations in the two other cases (UNGA 2013a). In September 2013, following negotiations with Russia and the United States of America (USA), Syria agreed to join the CWC and declared its chemical weapons stockpile for dismantling.

Reports of continued chemical weapons use re-emerged in 2014. In response, the OPCW Director-General established a Fact-Finding Mission (FFM) based on his general authority granted by the CWC. The FFM is the first ad-hoc mechanism based solely on OPCW’s authority with a mandate for establishing facts about the alleged use of chemical weapons without assigning responsibility (OPCW 2015). The mechanism was endorsed by decisions from the OPCW Executive Council and UN Security Council Resolution 2118. The FFM noted over 146 alleged cases of concern, investigated 71 allegations, and issued 20 reports concluding that in 39 instances, chemical weapons were used or likely used (OPCW 2023c).

Subsequently, in 2015, UN Security Council Resolution 2235 created the UN-OPCW Joint Investigative Mechanism (JIM) to build on the FFM’s work and assign responsibility for incidents confirmed by the FFM. While it can be stipulated that the CWC and UNSGMFootnote 2allow for some extent of attribution, neither of the previous investigations in Syria contained such a mandate and consequently the JIM was the first mechanism with such an explicit attribution function. From 2016–2017, the JIM investigated 9 out of the 23 cases of chemical weapons use confirmed by the FFM, and released 7 reports, in which it attributed responsibility to the Syrian government for 4 incidents from 2014–2017 and the terrorist group ISIS in two others (Secretary-General 2017; 2016b; 2016d; 2016c).

In 2017, the UN Security Council did not renew the JIM’s mandate. As a reaction, in 2018, the CWC Conference of States Parties, in a special session, tasked the OPCW Technical Secretariat with establishing the Investigation and Identification Team (IIT).Footnote 3 The IIT had the objective of attributing responsibility for cases “in which the FFM has determined that use or likely use of chemical weapons occurred and for which the JIM has not reached a conclusion on attribution” (OPCW 2018a). The OPCW broadly mirrored the JIM’s mandate through the IIT but without UN involvement. Out of the 33 eligible cases, the IIT investigated 9 cases and attributed responsibility in 5 cases (OPCW 2020a; 2021; 2023a), (Figs. 1 and 2).

Fig. 1
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An overview of the chemical weapons investigative mechanisms discussed in this paper and the activation period of each mechanism. The logos (of the OPCW and UN) indicate the establishing body for these mechanisms (Authors’ own work)

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Fig. 2
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Comparison between the work of the investigative mechanisms as of 2023. Black indicates the number of reported allegations of chemical weapons use. Dark gray indicates the number of cases and incidents investigated. Light gray indicates the number of cases and incidents which were confirmed or attributed following the investigative work. The dashed white bars indicate the number of deployments. Authors’ work based on data collected from the UN and OPCW (Latest data on the work of the FFM and IIT data are summarized on the OPCW webpages: https://www.opcw.org/fact-finding-mission and https://www.opcw.org/iit. Detailed list of the allegations under the FFM is provided in the Annex of the OPCW Technical Secretariat’s report “summary update of the activities carried out by the OPCW Fact-Finding Mission in Syria” dated 21 December 2023 (S/2232/2023). The data on the UNSGM and JIM were obtained from the official reports (UNGA 2013a; Secretary-General 2017; 2016b; 2016d; 2016c). The number for alleged cases under the Challenge Inspection mechanism is a conservative estimate based on the total number of allegations reported to the UNSGM and FFM. The number could be higher if allegations reported by non-official open sources are taken into account. For example the databases provided by the Global Public Policy Institute.)

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While having distinct mandates, each mechanism informed and influenced the work of subsequent mechanisms and paved the way for reinforcing the investigation norm (Kelle 2023). For example, FFM findings were instrumental to the JIM and IIT’s work, as both mechanisms were explicitly tied to the cases identified by the FFM. The JIM relied heavily on FFM data and evidence, which constituted a “starting point” for JIM’s work. JIM leadership board had meetings and technical exchanges with the FFM staff and its leadership (Secretary-General 2016a).Footnote 4 Moreover, some experts from the previous mechanisms were involved and supported JIM, some even in a formal position, such as the head of the UNSGM mission who served as Senior Scientific Advisor (Sellström 2021).

Similarly, JIM supported and informed the IIT. The JIM’s mandate and outcomes not only fed into the IIT’s work but also paved the way for the evolution of the attribution norm and the development of the IIT mandate within the OPCW. Being part of JIM, the OPCW benefited from JIM’s experience in developing procedures and methods for attribution purposes.

The mandate of other mechanisms on Syria, such as the Human Rights Council’s Commission of Inquiry and the International, Impartial and Independent Mechanism (IIIM), established by the UN General Assembly, also included activities related to chemical weapons use, either part of human rights violations or other violations of international humanitarian law. The IIT is indeed tasked to cooperate with the IIIM (OPCW 2018a, para. 12), which makes the applicable standard of proof a highly relevant question.

4 Standard of proof

Investigative and fact-finding mechanisms, such as those employed on an ad-hoc basis in the case of alleged chemical weapons use in Syria, have to make a determination on the required level of certainty applied to their findings. This level of certainty, also called standard of proof, derives originally from judicial proceedings but has become an accepted term in fact-finding, predominantly in human rights mechanisms.

A standard of proof only relates to the minimum level of certainty and does not preclude evidence of a higher degree of certainty from being included. It rather clears out any evidence that falls below the threshold determined in the standard of proof from being included and thereby helps in understanding the scrutiny that has been applied in order to reach conclusions.

As fact finding has evolved considerably over the past decades and has become increasingly standardized over time, different categories of standards of proof seem to emerge (Boutruche 2011). Following Wilkinson, who provides a comprehensive examination of these potential standards, the four categories referenced in this study are: Reasonable Suspicion/Reasonable Grounds, Balance of Probabilities, Clear and Convincing Evidence, and Overwhelming Evidence (Wilkinson 2014). Importantly, these are not clearly separable categories, authors may use different terms for the same standard (Ali et al. 2020), and oftentimes the evidence falls somewhere on the scale between different levels of confidence. Moreover, many mechanisms do not explicitly state their standard of proof, even although explicitly naming the applicable standard is recommended as best practice (Harvard Humanitarian Initiative 2015). In cases where such a standard is not explicitly mentioned, it can be deduced from the language used to describe the evidence and findings. Additionally, with progressing alignment of methods between different mechanisms, these categories also change over time, potentially resulting in the application of a lower standard of proof, as discussed more in depth later in this paper. As such, these categories serve as a general indication of the standard applied but should not be taken as a precise value.

Reasonable Suspicion is the lowest threshold, only requiring a sufficient likelihood that a violation took place, and it is the most commonly employed standard in fact-finding or in the pre-trial phase of judicial proceedings (OHCHR 2015; 2018).Footnote 5 The Balance of Probabilities standard requires that it is more likely than not that the violation took place. The Clear and Convincing Evidence standard goes further, requiring that every finding be made with a high degree of certainty, which makes this standard suitable to some cases of fact-finding missions but potentially unattainable for others. The Overwhelming Evidence standard corresponds to the common law term “beyond reasonable doubt,” which is the standard required in judicial criminal proceedings. Since fact-finding missions do not replace judicial proceedings, they rarely apply this standard but might choose to do so in specific cases (Fig. 3).

Fig. 3
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Differing Standards of Proof (adapted and adjusted from Wilkinson 2014)

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When analyzing the mandate and final reports of the UNSGM, FFM, JIM, and IIT, it is apparent that they employ differing standards of proof, even though, in some cases, the categorization is not clear cut.

The UNSGM does not explicitly state its standard of proof, but from its Preliminary Report in September 2013 and the Final Report in December 2013, it can be inferred that the highest degree of certainty reached was Clear and Convincing Evidence. The final report expresses in its methodology that it requires “sufficient and credible information to substantiate each allegation” to initiate an investigation (UNGA 2013a), which would constitute a lower threshold. However, both reports stated in their conclusions that they respectively found “clear and convincing evidence that chemical weapons were used also against civilians, including children, on a relatively large scale in the Ghouta area of Damascus on 21 August 2013” (UNGA 2013a and 2013b). This shows that the experts reported their findings with different degrees of certainty, ranging up to Clear and Convincing Evidence.

The OPCW FFM seems to apply a standard falling somewhere between Reasonable Suspicion to Balance of Probabilities. None of the numerous reports explicitly states the applicable level of certainty, but repeated formulations such as “reasonable degree of confidence,” “reasonable grounds to believe,” “reasonable grounds for the FFM to determine” and “reasonable grounds to indicate” correspond to the lower standard of Reasonable Suspicion (OPCW 2015, 2019a, 2020b, 2022, 2023d). Yet, in some instances, the reports also use language that might indicate a higher standard of Balance of Probabilities, for example, by formulating that the evidence “leads the FFM to conclude, with a high degree of confidence” (OPCW 2014). This exemplifies that the standards of proof are on a continuous scale, and the FFM applied a mixture of both standards by including evidence on the level of Reasonable Suspicion in its reporting but clearly indicating cases where they felt more confident in the evidence.

Similarly, for the OPCW-UN JIM, in the absence of an explicit mention of a standard of proof, it is necessary to deduce from the language used in its reports. Here most formulations, indicate that the desired standard is somewhere between Balance of Probabilities and Clear and Convincing Evidence, as exemplified by formulations such as “sufficient, credible and reliable evidence” and “evidence of a credible and reliable nature” (Secretary-General 28,29,a, b, 2017). However, one report also states that the different levels of evidence that will be considered are “overwhelming evidence,” “substantial evidence” and “sufficient evidence” (Secretary-General 2016d). This indicates that the JIM will certainly include evidence on the standard of Overwhelming Evidence but is also satisfied to draw conclusions with “sufficient evidence”, which corresponds to the lower Balance of Probabilities.

The OPCW IIT is the only mechanism examined in this paper that explicitly states its applicable degree of certainty, which is “reasonable grounds to believe.” In one of its reports, it is stated that this is akin to the Reasonable Suspicion standard used in other mechanisms (OPCW 2020a).

In comparison, the International, Impartial and Independent Mechanism (IIIM), which is not an investigation mechanism but rather tasked with preserving evidence for national judicial proceedings, including into chemical weapons use in Syria, declares that it applies the highest standard, i.e. Overwhelming Evidence, as it explicitly applies the criminal law standard of “beyond reasonable doubt” (International, Impartial and Independent Mechanism 2019). Since the evidence produced by the IIT generally adheres to a lower standard, not all evidence collected might be suitable for transfer between the two mechanisms. Importantly, this does not preclude evidence from being utilized by the IIIM, but might require additional steps of verification, for example by cross-referencing with other sources.

A comparison between the standards applied in investigations into alleged chemical weapons use in Syria shows that the minimum standard applied is a relatively high degree of Reasonable Suspicion for the IIT, and the highest standard is Clear and Convincing Evidence for the UNSGM and partly for the JIM, which does also include evidence on the level of Balance of Probabilities. The FFM seems to seek evidence that falls somewhere between Reasonable Suspicion and Balance of Probabilities.

5 Investigative working methods and considerations

In investigating the alleged use of chemical weapons in Syria, the UNSGM adhered to its endorsed and established institutional guidelines and procedures as the basis for conducting the investigation, but also adopted other operational procedures from the OPCW. These include procedures for interviews, chain of custody and evidence collection and analysis. In specialized areas, such as epidemiological investigation, UNSGM applied the scientific standards adopted by the World Health Organization (WHO) and OPCW to its work. The purpose of the mission and the existence of a cooperation framework between the OPCW and the United Nations enabled the involvement of OPCW’s experts and resources in conducting the investigation.Footnote 6 Furthermore, the analysis of collected samples was performed at OPCW-designated laboratories. These are accredited laboratories designated in accordance with decisions of the CWC State Parties.

Similarly, as the FFM operates within the CWC framework, it has followed the OPCW guidelines and procedures for investigating alleged use. Hence, it also relied on the designated laboratories for analyses. Unlike UNSGM and FFM, which already had procedures in place, JIM had to develop procedures further because there were no precedents for attribution mechanisms in this specific field of fact finding. As JIM is a joint mission of the OPCW and the United Nations, where the OPCW name takes precedence, the OPCW procedures and processes formed the basis for the newly developed guidelines and procedures, which were approved by the Security Council before the investigation started. As the IIT operates under the CWC, it has also had to develop its own procedures, building on the OPCW’s existing guidelines and procedures and drawing on other best practices to develop working methods further, especially for new areas where expertise is required. These procedures were then approved by the State Parties and included instructions and practices for conducting investigations and collecting evidence, information management, chain of custody, and documentation (OPCW 2019a). While it is not clear to what extent these procedures differ from JIM guidelines and procedures, it is very likely that the OPCW’s participation in JIM has informed the development of the IIT methods.

The UNSGM did not accept evidence collected from external sources outside the mechanism’s expert monitoring. Information provided by states or relevant external parties, such as opposition representatives, was only considered if UNSGM was able to independently verify its credibility (UNGA 2013a). In contrast, FFM and IIT were more open to data provided by external sources, including publicly available open source information. However, sources for which the chain of custody or credibility could not be verified were not accepted.

While the FFM’s data constituted the primary source for the JIM, it also gathered data from states, intergovernmental organizations, open sources, and other non-governmental actors, mainly for the planning and initial stages. However, the JIM commissioned forensic institutions recognized by the United Nations to verify the authenticity and credibility of data provided by these sources (Secretary-General 2016c), (Table 1).

Table 1 Summary attributes for the four investigation mechanisms including the mandates, objectives and outcomes, methods and technology
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Securing access to the locations of alleged incidents was cited as the biggest challenge for most mechanisms. In the early stages of FFM work, access constraints were due to safety concerns as the sites were in areas of armed conflict. However, at a later date, and similar to the case of the IIT, access restrictions were attributed to a lack of cooperation from the Syrian government. JIM also reported a lack of access to locations, although the reason given was different from FFM and IIT. According to JIM reports, access was not denied by any party, but in some cases the team could not get to the locations for security and safety reasons.

Other challenges reported in the collection of evidence included the time lag between the incident and the investigation which in some cases led to the loss of potential evidence or its integrity. For example, chemicals such as chlorine degrade rapidly, so that the probability of detecting them after a certain time decreases. These time challenges can also lead to the scene being changed, including through cleanup, staging or the alteration of evidence. Another challenge was access to verifiable information and the credibility of sources provided by parties to the conflict. Plausibly, in certain cases, such as in the work of FFM, this made it necessary to turn to additional sources of information, including open sources.

Nevertheless, the mechanisms show pragmatism in applying their working methods in an attempt to overcome some of these limitations. In this regard, the JIM has taken a pragmatic approach in several cases. First, it did not have to start from scratch. It relied on massive, ready-to-use, credible data and evidence provided by FFM, which constituted the basis of its resources. Second, the JIM’s founding body and mandate ensured cooperation with states for obtaining information, technical and logistical support and access. This was guaranteed by the United Nations Security Council (UNSC) resolution 2235, which required states to cooperate. Third, JIM took a strategic approach in establishing its working methods. The selection and prioritization of cases to be investigated was based on factors that would increase the chances of achieving more conclusive findings, including the quantity of available data, “forensically verifiable information”, “consistency and coherence among accounts given by sources”, and the ability to “independently verify and corroborate already available information.” (Secretary-General 2016a).

The same favorable circumstances cannot be assumed for the IIT, not only due to the cooperation and access it did enjoy but also given its restrictive mandate. The IIT attempted to follow the same criteria for selection and prioritization. However, because the IIT was mandated to investigate cases in which the JIM did not reach a conclusion on attribution, it had to deal with more complicated cases or incidents that scored relatively poorly on the above factors. It is therefore plausible to argue that IIT’s endeavor to reach more conclusive findings was more difficult compared to the JIM.

6 Role of technology and its limitations

Incorporating advanced and tested technology into working methods facilitates the work of investigation mechanisms. It enables them to retrieve more evidence of higher quality and achieve their goals even in challenging situations without compromising the safety of personnel, the integrity of collected evidence and samples and the credibility of the results. For instance, inspectors can use digital forensics technology such as photogrammetry, chemical dispersion simulation or munition trajectory modeling to reconstruct possible scenarios or past events that occurred in locations not accessible to the investigation team for safety or security reasons (OPCW 2019b). This is also useful to overcome some of the challenges that arise due to the time lag between the incident’s occurrence and the investigation.

The extent to which technology was utilized in the working methods of the four investigation mechanisms varied from one mechanism to another. The mechanisms investigating the alleged use without attributing responsibility, the UNSGM and the FFM, relied primarily on the analysis of environmental samples (soil, rubble, fabric, etc.) and biomedical samples from human sources (blood, hair, organs and tissues, etc.). These analyses aimed at confirming the presence of chemical signatures or traces (UNGA 2013a).

On the other hand, attribution mechanisms JIM and IIT utilized additional technology including chemical forensics, analysis of meteorological data and satellite images, ballistics analysis, and munition analysis. All of this contributed to achieving the mechanisms’ goal of determining and assigning responsibility for chemical weapons use (OPCW 2020a; 2023a ; Secretary-General 2016b).

Depending on the objective of the mechanism, technology could increase the chances of achieving more conclusive findings. For instance, chemical forensics, which allows chemical agents and by-products to be traced back to their source by comparing chemical fingerprints with profile data stored in verified databases, could confirm conclusions about suspected perpetrators or at least narrow down the list of suspects (OPCW 2023a).

However, there is a limit to what technology can contribute. For instance, chemical profiling is one of the key forensic tools used for determining attribution signatures. The results provide information on how chemicals were produced and where they originate from. This is done by determining the presence and abundance of by-products and impurities of chemical agents. However, the technology findings may not be useful without reference databases and accurate historical or contextual information, such as production methods and formulas, against which to compare the results (Höjer Holmgren et al. 2023).

Furthermore, depending on the objectives of the mechanism and the resources available, not all technologies are applicable or suitable. For instance, it was not plausible for the UNSGM or FFM to invest time and resources in using chemical forensics to investigate the origin of a chemical if they were not mandated to attribute it. Therefore, it is obvious that the mandate, i.e., whether the attribution of responsibility is expected, is the strongest factor in determining whether additional technology, such as chemical forensics, is applicable and could be employed.

Finally, another reason why it is difficult to determine the extent of technology’s impact on the standard of proof may be lack of available information and understanding of technology’s capabilities. While it seems more accurate to make an in-depth comparison between mechanisms with similar mandates (i.e., compare UNSGM with FFM and JIM with IIT) to draw a conclusion about the impact on the standards of proof, this may be difficult due to the limited data. For instance, there is insufficient information on the exact technology used to perform forensics analyses in the JIM’s work. This would make it difficult to compare forensic technology of the JIM with that of the IIT. In a “lessons learned” report on the JIM, the researchers found that attention must be given to “how much detail to expose in … methodological considerations” (UNODA 2020). Transparency must be balanced with considerations of operational security, including the possibility of jeopardizing ongoing investigations. Nevertheless, the inclusion of chemical forensics in investigations of suspected use is a relatively new field that is still rapidly evolving. Therefore, it is plausible to suggest that the level of technological forensics capabilities that were available to the JIM differed from those that the IIT later possessed.

7 Factors and attributions influencing the standards of proof

Since the JIM and IIT as attribution mechanisms have a broader mandate and therefore employed additional means of technology compared to the UNSGM and FFM, they are expected to provide a higher degree of certainty than the mechanisms that do not pursue attribution. However, the findings of this paper do not support this hypothesis. The highest standard is applied by the UNSGM, i.e., Clear and Convincing Evidence. This is followed by the JIM which applies this standard as well as Balance of Probabilities. The FFM applies Balance of Probabilities and Reasonable Suspicion, while the IIT mainly pursues Reasonable Suspicion.

It appears that the distinguishing variable between the mechanisms is not related to attribution or the advanced technologies they use. Instead, the different standards of the various mechanisms can be explained by three main factors: the mandating body, the circumstances surrounding the investigation, and the reporting of the findings.

The UNSGM and the JIM were both mandated by the UNSC, which is the main political body with binding decision-making power in international relations. In contrast, the FFM and the IIT were mandated by the OPCW, a primarily technical organization that does not explicitly provide for the creation of such ad hoc mechanisms in its convention. The findings of the UNSGM and the JIM were submitted to a forum that has the mandate to decide directly on possible consequences. The UNSC has the power to pass legally binding resolutions that could have punished those responsible for the use of chemical weapons (Jakob 2019). The availability of such follow-up measures can be one factor in deciding to apply a higher standard of proof. In contrast, for the FFM and the IIT, the OPCW cannot pass any legally binding resolutions that go beyond its internal affairs, but must refer the matter to the UNSC.Footnote 7 The different objectives between fact-finding without attribution of responsibility on the one hand and investigations with attribution of responsibility on the other seem to play a lesser role than the actual capacity of the mandating body to take punitive measures. An exception is the IIIM, which applies a significantly higher standard of proof, even if it cannot itself initiate criminal proceedings. However, as this mechanism is designed to support and simplify national criminal prosecutions, the focus on a streamlined standard of proof may override any concerns about access, mandating power or ability to pass punitive measures itself.

The mandate and the mandating body influence the standard of proof by creating minimum expectations for the mechanisms to achieve. However, other elements related to the nature of the mandating body and the circumstances surrounding the establishment of the mechanisms and the conduct of investigations also play a role. This is related to the second factor that explains why, for example, the UNSGM and the JIM apply a higher standard of proof: external circumstances such as political support, cooperation, and granted access to information and sites. Both the UNSGM report and the first JIM report emphasized expectations for cooperation with the Syrian government in providing access to investigation sites (UNGA 2013a, Secretary-General 2016a) and were able to conduct at least some of their investigations on-site. Since both mechanisms are mandated by the UNSC, they had at least tacit support from Syria’s close ally, Russia.

The variation in political support can also be observed in the decision-making and voting procedures for establishing these mechanisms. While the decision to establish the JIM at the UNSC was made unanimously and could have been blocked by a veto from any of the five permanent members, the IIT was created through a majority vote at the OPCW. Although legally in line with the prescribed voting procedures, a non-consensual decision may carry less political weight compared to an unanimous UNSC decision. This may explain why some states questioned the mandate to establish the IIT early on. Consequently, the fact that the UNSC’s mandating power is clearly stronger and less contested might influence the degree of certainty that these mechanisms seek to attain.

In contrast, the OPCW mandated mechanisms, especially the IIT, have been strongly opposed by both Russia and Syria, and most reports have cited the lack of access to sites of interest as the main obstacle (OPCW 2019a, 2020a, 2021, 2023a). This lack of access was to some extent predictable, as Russian and Syrian diplomats stated from the outset that they do not deem the IIT legitimate (OPCW 38,39,c, d). It is plausible to argue that due to this lack of access, the degree of certainty was lower than that for the mechanisms where there was at least the possibility of better access.

Another relevant factor is reporting. While the mandating body sets the minimum expectations and the circumstances of the work of the mechanisms influence how these expectations are met, it is the reporting that shows how these expectations are met. The way in which mechanisms report on evidence and findings is key to demonstrating how the standard of proof is achieved. Practically, this means the ability to calibrate the report’s vocabulary so that the findings are framed in terms of the standard of proof, i.e. that adequate information is provided to support the rationale and conditions for the reported level of certainty and to explain how the evidence is assessed, factual conclusions are drawn from findings, facts are classified and legal concepts and rules are applied to findings. This includes reflecting on situations where the findings exceed, or fall short of, the level of certainty by explaining the constraints and limitations of the investigations and the circumstances under which evidences is collected, taking into account the expectations set by the mandate.

In the field of fact-finding, a trend towards greater standardization can be observed following the publication of new resources on standard best practices and guidelines after 2015, such as the United Nations publication Commissions of Inquiry and Fact-Finding Missions on International Human Rights and Humanitarian Law: Guidance and Practice. These guidelines recommend a more deliberate and explicit approach to the application of standards of proof and the formulation of evidence in reporting (Harvard Humanitarian Initiative 2015). It is plausible to assume that this increased standardization, combined with lessons learned from previous mechanisms, has informed subsequent investigations and influenced the selection and reporting of standards. For instance, the early establishment of standard methods for the mechanism’s work, including for reporting, and their dissemination to stakeholders, as well as the crucial role of legal experts in developing these standards and advising on reporting, were identified as key lessons learned from the JIM experience (UNODA 2020). Indeed, the newest investigative mechanism examined in this study, the IIT, is the only one that explicitly mentions its standard and opts for the application of the Reasonable Suspicion standard, drawing on standard practice and the generally accepted approaches of international fact-finding bodies and commissions of inquiry (OPCW 2020a).

The explicit choice of the lowest standard by the IIT exemplifies a certain degree of pragmatism in presenting the findings through a lower standard of proof. In light of an increasingly challenging situation for investigations, particularly access for on-site investigations, a higher standard might not always be achievable, but the evidence might still be relevant and thus published with the clear emphasis on the lowered degree of certainty. This is in line with a general trend in fact-finding in recent years, where missions are increasingly adopting a lower standard to better suit their mandate and likely access. Finally, as mentioned above, this does not preclude the IIT from also making findings with a higher degree of certainty, with the explicit mention of the applicable standard contributing to greater transparency of the reports.

8 Conclusion

This paper has presented a comparison between the four investigative mechanisms on the alleged use of chemical weapons in Syria, focusing specifically on the technology and working methods used and the standard of proof applied. The comparison has shown that those mechanisms that contain a mandate to attribute responsibility, i.e., the JIM and IIT, use a wider range of technologies, such as chemical forensics, meteorological and satellite imagining. However, this increased use of technology was not accompanied by a higher standard of proof.

Instead, this paper finds that the standard of proof depends significantly on the mandating and founding body of the mechanisms, as well as the circumstances surrounding the investigations, such as political support and the anticipated and granted level of cooperation and access to investigation sites. The paper also mentions a third possible factor influencing the standard of proof. The minimum standard of proof is set according to the mandate’s expectations, and the aforementioned circumstances influence whether investigations can conclude with findings that meet, fail to meet, or exceed these expectations. However, it is the ability to report the findings in a conclusive and legal manner that demonstrates the achievement of a specific standard of proof.

The paper shows that the UNSGM and the JIM, mandated by the UNSC, apparently expected better access and cooperation, and therefore generally sought higher standards of proof compared to the OPCW-mandated FFM and IIT. The fact that the findings of UNSGM and JIM had the potential to lead to direct punitive measures through the UNSC may also have contributed to a higher standard of proof. The standard of proof thus is influenced both by the expected and actual access and by the expected follow-up requirements, such as the use of reports and findings in future punitive and judicial proceedings.

However, it should be noted that the standard of proof sought by the mechanisms only creates a minimum degree of certainty. A lower envisioned standard should thus not be equated with a lower quality of evidence, and especially the IIT’s quality of evidence often exceeds the envisioned standard. For instance, the fact that the IIT was the only mechanism to explicitly state its applicable standard has to some extent contributed to greater transparency and is in line with best practice in fact finding.

Lastly, while the paper covers various aspects of the application of standards of proof in meeting the objectives of investigation mechanisms and discusses correlations with relevant factors such as methods, mandate and technology, the paper invites further future research on this topic, including research in areas that have not been explored extensively such as the role of other relevant factors such as time-limited mandate or level of attribution (State Party, unit, individual), researching the variation in applying standards of proof in different areas, including in human rights and war crimes investigations, and further implications of the application of different levels of standards of proof on the finality of investigation’s outcomes (i.e. follow-up punitive and judicial actions).