In 2012, as armed conflict raged in Syria, the Assad regime gave assurances it would never use chemical weapons against its own people, "no matter what the internal developments in this crisis are".

Speaking at a news conference, foreign ministry spokesman Jihad Makdissi insisted the country's chemical weapons were intended only for national defence and there was also no chance of them falling into the wrong hands: "All varieties of these weapons are stored and secured by the Syrian armed forces and under its direct supervision, and will not be used unless Syria is subjected to external aggression."

At the time, Syria – along with Egypt, North Korea and South Sudan – was one of only four countries that had not signed the Chemical Weapons Convention, the international pact banning possession and use of such weapons, and was widely believed to have been secretly developing them – mainly in response to Israel's nuclear programme. Makdissi's statement, though, was the first official confirmation that Syria did in fact possess chemical weapons.

Since the spring of 2013 numerous chemical attacks have been documented in Syria. The regime denies any responsibility for them and, at first sight, seems eager to identify the culprits. However, its attitude towards international investigations tells a different story – one which is scarcely suggestive of innocence.

Renouncing chemical weapons

In August 2013, hundreds of people died when rebel-held areas of Ghouta, on the outskirts of Damascus, were attacked with the nerve agent, sarin. This was the most serious chemical attack in Syria to occur so far and it caused an international outcry. Suspicion fell on the regime and, under the threat of US military action, it agreed to join the Chemical Weapons Convention.

By joining the convention, Syria committed itself to chemical disarmament. It was required to declare all its stocks and related production facilities, which would then be destroyed or dismantled under supervision of the international Organisation for the Prohibition of Chemical Weapons (OPCW).

The start looked promising. Syria wasted no time in submitting its declaration and, in a speech to the OPCW's executive committee, assistant foreign minister Husamuddin Alla boasted that it had done so four days ahead of the legal deadline. This, he said, was a sign of Syria's "full commitment" to complying with its obligations under the convention.

It wasn't long, however, before the OPCW's assessment team began finding errors and omissions in the Syrian declaration. Some of them might be attributed to initial confusion about what information was required under the complicated rules of the convention but, almost four years later, despite numerous amendments to the original declaration, the OPCW has still not accepted it as complete. There remain "gaps, inconsistencies and discrepancies" (to use the OPCW's phrase) which have yet to be explained.

While it might be expected that the number of unresolved issues would have been whittled down over time, in Syria's case the opposite happened. According to the OPCW, the number has "steadily increased" – along with the assessors' scepticism.

In a blistering report last year, the OPCW's director-general, Ahmet Üzümcü, complained that many of Syria's answers to questions were "not scientifically or technically plausible". In many instances, he said, new information proffered by Syria "presents a considerable change in narrative from information provided previously – or raises new questions. In some cases, this new information contradicts earlier narratives."

Gaps in the regime's declaration

Full details of Syria's declaration have not been made public but documents on the OPCW's website show the regime disclosed 41 chemical facilities at 23 sites:

• Eighteen chemical weapons production facilities, including some for filling weapons
• Twelve chemical weapons storage facilities, consisting of seven reinforced aircraft hangars and five underground structures
• Eight mobile units for filling weapons
• Three other facilities related to chemical weapons

As far as actual weapons were concerned, the regime declared:

• 1,000 tonnes of Category 1 chemicals
• Approximately 290 tonnes of Category 2 chemicals
• Approximately 1,230 unfilled chemical munitions
• Two cylinders, later found to contain sarin, which the Syrian authorities said did not belong to them

(Category 1 means "high risk" chemicals, including nerve agents, which are considered especially hazardous and which have little or no use outside chemical warfare. Category 2 includes "significant risk" chemicals such as phosgene which also have some legitimate civilian uses.)

Despite Syria's claim of making "all possible efforts" to fulfil its obligations, there is now no doubt the declaration was incomplete – though how incomplete remains an unanswered question.

Syria had initially declared four different chemical warfare agents but, following "consultations" with the OPCW, it added a fifth to the list. Later, tests on samples obtained by inspectors in Syria "indicated potentially declarable activities involving five additional chemical agents". More "consultations" ensued and the list grew to six. The most recent information from the OPCW is that there are still four chemical agents detected through sampling whose presence the regime "has not yet adequately explained".

Discrepancies and difficulties

There were also discrepancies between Syria's records of chemical weapons production and the quantities it had declared. The OPCW said it was "unable to verify the precise quantity of chemical weapons that were destroyed or consumed" before Syria signed up to the convention. At one point Syria claimed it had used 15 tonnes of nerve agent and 70 tonnes of sulfur mustard for research – figures that the inspectors found hard to believe since only tiny amounts would be needed for research.

On the munitions front, there were difficulties accounting for 2,000 or more chemical shells which Syria said it had either used or destroyed – after purportedly adapting them for non-chemical purposes. Again, the inspectors doubted the truth of this because converting the shells into conventional weapons would scarcely have been justified by the effort and expense.

In August 2014 the OPCW certified that all the Category 1 chemicals declared by Syria (primarily nerve agents and their precursors) had been destroyed. That, of course, did not resolve the question of whether Syria still had undeclared stocks or production facilities – a question that became especially pertinent following the sarin attack in April this year that killed dozens of people in Khan Sheikhoun.

Aside from questions about the declaration, inspectors working on the ground in Syria faced various difficulties which might – to put a charitable spin on them – be attributed to the foibles of Syrian bureaucracy but which cumulatively gave the impression of intentionally hampering investigations.

According to a report by Anthony Deutsch of Reuters, these have included "withholding visas, submitting large volumes of documents multiple times to bog down the process" and last-minute restrictions on site inspections. Deutsch describes one example of these restrictions in 2015 that aroused inspectors' suspicions:

A Syrian major general escorted a small team of chemical weapons inspectors to a warehouse outside the Syrian capital Damascus. The international experts wanted to examine the site, but were kept waiting outside in their car for around an hour, according to several people briefed on the visit.

When they were finally let into the building, it was empty. They found no trace of banned chemicals.

"Look, there is nothing to see," said the general, known to the inspectors as Sharif, opening the door.

So why were the inspectors kept waiting? The Syrians said they were getting the necessary approval to let them in, but the inspectors had a different theory. They believed the Syrians were stalling while the place was cleaned out. It made no sense to the team that special approval was needed for them to enter an empty building.

Deutsch's report also hints at obstruction during some of the interviews conducted by inspectors:

In Damascus, witnesses with knowledge of the chemical weapons programme were instructed by Syrian military officials to alter their statements midway through interviews with inspectors, three sources with direct knowledge of the matter told Reuters.

There are indications, too, that inspectors have not been allowed to interview the most relevant people. In his 2016 report, Üzümcü urged the authorities in Damascus to "facilitate access to individuals with strategic knowledge and oversight" of the chemical weapons programme.

A new twist: chlorine bombs

While struggling to verify Syria's declaration, the OPCW has also been working separately through its fact-finding missions to investigate reported chemical attacks in Syria, and a UN-OPCW panel known as the Joint Investigative Mechanism has been assigned the task of identifying perpetrators.

Internationally, it is the deadly nerve agent sarin that has caused the most alarm but sarin is not the only chemical at issue. Chlorine has been used “systematically and repeatedly” according to the OPCW and sulfur mustard – commonly known as mustard gas – has been used to a lesser extent too. At least one of the mustard attacks, at Um Housh a year ago, was blamed by locals on ISIS.

Chlorine is a common substance with multiple civilian uses but is classed as a chemical weapon when used in warfare. Although less toxic than sarin, it irritates the eyes and skin, and can cause permanent lung damage. In high concentrations it can be lethal.

The regime appears to have begun chlorine attacks in 2014, just a few months after officially renouncing chemical weapons. Numerous witnesses have described chlorine bombs being dropped from helicopters – in which case rebel groups could not be responsible since they have no aircraft.

Furthermore, the OPCW was able to confirm the air-dropping of chlorine bombs. From photographed remnants of the bombs it established that they were designed as free-fall weapons, with stabilising fins so that the detonator section would hit the ground first. There was no sign of a propulsion system and they were judged too large to be launched from land-based artillery systems.

Thus the regime's use of chlorine is beyond reasonable doubt – which puts it in breach of the Chemical Weapons Convention. But chlorine doesn't arouse the same kind of international horror as sarin and the OPCW has pursued the issue less vigorously. One reason is that chlorine bombs came into use at a time when dismantling Syria's declared chemical stockpile was seen as the top priority and there were fears that a new confrontation over chlorine could jeopardise that process.

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Feeding scepticism

If, as the regime claims, it was not responsible for any of the chemical attacks and has been honest in its declaration to the OPCW, it has not tried very hard to demonstrate its innocence. In a way, though, it doesn't need to. It is protected by Russia in the UN Security Council and where chemical weapons are concerned the American appetite for unilateral military action appears to be limited. Trump's response to the sarin attack in Khan Sheikhoun last April – firing dozens of cruise missiles at Shayrat airbase, the alleged source of the attack – was certainly dramatic but of dubious effectiveness. The missiles cost US taxpayers around $49m but the cost to the regime was relatively insignificant and the base was reportedly operational again very swiftly. If anything, the bombing of Shayrat may have encouraged the regime to step up its use of chlorine.

In the propaganda war, the regime also benefits from the Iraq effect. Regardless of how much evidence the OPCW produces, significant sections of the public – some of them very active on social media – remain sceptical. Although the regime is known to have possessed sarin and there is no credible evidence that rebel groups ever had access to it, the deception by western governments in 2003 over Saddam Hussein's non-existent weapons of mass destruction still makes people wary.

The regime has exploited this with a media strategy which is aimed at feeding people's doubts rather than putting forward a coherent narrative in its defence. The basic technique – in which Russia has also been very active – is to promote "alternative" theories, often plucked from unreliable sources on the internet, and then accuse the UN/OPCW of failing to investigate them seriously.

It doesn't much matter if the "alternative" theories conflict with each other and are not supported by credible evidence, so long as they can be kept in circulation. The point is not really to convince anyone that they are true but to neutralise public opinion. If multiple theories can be kept floating around in the mix, the idea that the regime attacked its citizens with sarin starts to look like one possible explanation among many and, with luck, people will be unsure which of them – if any – to believe.

Claims of faked videos

In 2013, two days after the Ghouta attacks, Russian foreign ministry spokesman Aleksandr Lukashevich asserted that YouTube videos of victims posted on the internet were fakes. The idea had come from a pro-Hizbullah website which wrongly claimed that the videos in question (along with a Reuters report of the attack) had appeared the day before the attack took place.

The apparent discrepancy in timing was a result of different time zones around the world – YouTube videos are automatically stamped with the local time in California which is 10 hours earlier than Damascus time – but that did not put paid to the claims about faked videos. Shortly afterwards, Mother Agnes, a Syrian-based nun in the Melkite Catholic church who was sympathetic towards the regime, issued a report disputing 13 of the videos.

"The civilian population in East Ghouta as presented in those videos is inconsistent with the composition of a real civilian society", she wrote. "There is a flagrant lack of real civilian families in East Ghouta, as presented by the videos."

Mother Agnes claimed that some of the children seen in the videos were not from Ghouta but had been kidnapped by jihadists earlier in the summer from pro-regime villages in other parts of the country.

Three of the videos, she said, showed "artificial scenic treatment using the corpses of dead children", while in others she suggested the children were anaesthetised or merely sleeping. Human Rights Watch dismissed Mother Agnes's claims as baseless but the Russian propaganda channel, RT, continued to treat her as a quotable source.

Alleging that images of the victims were faked implied there were no real victims to film, and on that point Assad supporters initially hedged their bets. Lukashevich, for example, talked of an "alleged" attack and a "so-called" attack on Ghouta while citing the supposedly premature videos as evidence of "a pre-planned action" which signalled the launch of "another anti-Syrian propaganda wave". Similarly, Mother Agnes talked of both "a forged story and a false flag".

The idea that the Ghouta attacks might have been invented by rebels became difficult to sustain amid growing evidence of real sarin use, however. Denying that it was the regime's sarin meant blaming the rebels instead – which in turn required some explanation as to why they would have massacred their own people.

Here again, the false flag theory came into play but another suggestion that excited Russian officials for a while was that the deaths had simply been an accident. Little more than a week after the attacks Mint Press, a US-based website, published a story claiming the deaths were a result of rebels mis-handling chemical weapons supplied to them by Saudi Arabia. According to the story, the Saudis had not informed the rebels they were chemical weapons or given instructions on how to use them. The only evidence to support this claim was a series of quotes from unidentifiable people in Damascus.

Accident theories surfaced again in 2017 when Syrian and Russian officials claimed the regime had not intentionally released sarin at Khan Seikhoun but that the Syrian air force had hit rebel-held chemicals during a bombing raid. Syria's foreign minister, Walid Muallem, said the raid had targeted a depot used by Jabhat al-Nusra which contained chemical weapons. Russian defence ministry spokesman Igor Konashenkov elaborated, saying the site included "workshops for manufacturing bombs, stuffed with poisonous substances".

As an explanation for the deaths caused by sarin, this was scientifically implausible. Even if the rebels had access to sarin (and there is no reason to think they did) they are unlikely to have held stocks of sarin as such. It is highly corrosive and degrades quickly in storage, so it is usually stored in binary form – as two separate components which are not mixed until just before use. Also, any actual (ready-mixed) sarin that was hit by a bomb would very probably be destroyed in the explosion. A UN report commented:

"It is extremely unlikely that an air strike would release sarin potentially stored inside such a structure in amounts sufficient to explain the number of casualties recorded. First, if such a depot had been destroyed by an air strike, the explosion would have burnt off most of the agent inside the building or forced it into the rubble where it would have been absorbed, rather than released in significant amounts into the atmosphere. Second, the facility would still be heavily contaminated today, for which there is no evidence."

'Clear and convincing evidence'

Within a month of the Ghouta attacks a report from the UN investigative team headed by Professor Åke Sellström informed the Security Council there was "clear and convincing evidence that surface-to-surface rockets containing the nerve agent sarin" had been used.

At a news conference, Russian foreign minister Sergei Lavrov acknowledged the use of chemical weapons but said Russia still suspected rebel forces were behind it: "We have very serious grounds to believe that this was a provocation."

He added that the investigators' report did not answer a number of questions Russia had asked, including whether the weapons were produced in a factory or home-made. The report, he said, should be examined not in isolation but along with evidence from the internet and other media, including accounts from "nuns at a nearby convent" [i.e. Mother Agnes] and a journalist who had spoken to rebels [i.e. the Mint Press article).

Assad: 'Any rebel can make sarin'

In an interview with Fox News a day after the UN report appeared, President Assad remained ambivalent about the events in Ghouta. Asked if he agreed with the UN's assessment "that a chemical weapon attack occurred on the outskirts of Damascus on August 21", he replied cryptically: "That's the information that we have, but information is different from evidence."

Pressed further, Assad said he was not disagreeing with the UN report but "you have to wait till you have evidence. You can agree or disagree when you have evidence." To Fox News's comment that the UN had already provided the necessary evidence, he responded: "We have to discuss it with them, we have to see the details."

Ignoring the results of laboratory tests, he went on to suggest – as Mother Agnes and the Russians had done earlier – that videos showing victims of the attacks had been faked.

"No-one has verified the credibility of the videos and the pictures ... You cannot build a report on videos if they are not verified, especially since we lived in a world of forgery for the last two years and a half regarding Syria. We have a lot of forgery on the internet."

On the question of who might be responsible for the attacks, Assad then made the startling claim that "any rebel can make sarin".

"Sarin gas is called kitchen gas," he said. "Do you know why? Because anyone can make sarin in his house."

Alternatively, he suggested, rebels could have got sarin from abroad: "We know that all those rebels are supported by governments, so any government that would have such chemical material can hand it over to those."

As far as home-made sarin is concerned, it's not a realistic proposition – especially in the quantities used for the Ghouta attacks – and so far no one has produced any credible evidence of rebels receiving sarin from abroad.

Åke Sellström, the chief UN weapons inspector, was puzzled that Syrian authorities persisted with these theories without producing evidence to support them. Interviewed in 2014, Sellström said:

"Several times I asked the government: can you explain – if this was the opposition – how did they get hold of the chemical weapons?

"They have quite poor theories: they talk about smuggling through Turkey, labs in Iraq and I asked them, pointedly, what about your own stores, have your own stores being stripped of anything, have you dropped a bomb that has been claimed, bombs that can be recovered by the opposition? They denied that.

"To me it is strange. If they really want to blame the opposition they should have a good story as to how they got hold of the munitions, and they didn’t take the chance to deliver that story.

Strange as that might have seemed to Sellström, it was perhaps only to be expected. Clarity is what the investigators seek but for the regime obfuscation can be a lot more useful.