The images from the apparent chemical weapons attack in Khan Sheikhun, in the Idlib province of Syria, are horrific. In the absence of confirmed scientific analysis, the signs of uncontrolled twitching, laboured breathing, excessive salivation and pinpointed pupils do suggest an organophosphate poisoning. Within that group of chemicals are the infamous nerve agents; so-called because of their mechanism of disrupting the nerve pathways. Chances of survival are dependent on many factors including the dose received, the speed of decontamination and the use of specialist injectable antidotes.

These latest attacks appear very different from the all too regular chlorine attacks reported from Syria over the last two years. While chlorine can be smelt and is lethal in high doses, it does not in any way have the toxicity and insidiousness of nerve agents. The numbers of people affected in Idlib this time do suggest something much more deadly.

Two scenarios have emerged on the source of the gassing. Opposition groups claim that the area was subject to a chemical attack from the air. The US and its allies broadly support this assertion, laying the blame on the Assad regime. However, the Syrian government and Russia insist that the agent came from an opposition weapons’ storage dump.

Either way, there are some serious issues of concern.

If there was indeed an attack by the Russian-backed Syrian air force, then clearly the international community, in the form of the UN security council, must attempt to take forward an effective response. But the options could be hindered by a Russian veto, last used only a few weeks ago to stop UN sanctions on Syria.

If the source is found to be nerve agent from the Syrian air force, then not only is this a clear war crime, but also leads to the question: how much chemical agent did Syria omit from its declaration to the UN?

In 2013 I led the operations to verify and disable Syria’s declared chemical weapons programme; a significant task given both the size of the stockpile and the environment in which the inspections were to take place. Syria had agreed to the inspections after a UN/OPCW/WHO investigation had proved that nerve agent had been used in the attacks around Damascus in August of that year.

The inspection timetable was tight; just 30 days to verify everything and disable the equipment so that no more weapons could be made. Our international teams spent very long days, sometimes in very dangerous circumstances, to get the task completed. The Syrian government clearly wanted to demonstrate their willingness to comply with the agreement. But the situation is different now. Syrian government forces now have the initiative and opposition groups are growing weaker – and again we see images of a poisoned and frightened population.

Lack of confidence in the Syrian declaration was expressed by the director general of the Organisation for the Prohibition of Chemical Weapons (OPCW) last summer after Syria submitted amendments to parts of its declaration.

He went on to state: “In particular, the lack of original documentation and access to senior leadership within the Syrian chemical weapons programme has precluded the secretariat from understanding the full scope of activities. In addition, some explanations were not scientifically or technically plausible.”

The doubts remain. As recently as March 2017, the UK delegation to the OPCW stated: “The extent of the gaps in Syria’s declaration, its cancellation of a planned visit by inspectors to the SSRC (Syrian government research laboratories that are suspected to have been involved in the development of Syria’s WMD programmes) ... and the ongoing use of toxic chemicals as weapons leaves substantial grounds for concern that Syria has retained elements of its stockpile and is prepared to use them again.”

Russia’s claim that the latest poisoning is a result of a conventional attack on an opposition arms storage facility should not be dismissed out of hand. While it is true that nerve agent can be destroyed by explosion, it is perfectly possible that some agent could survive and be ejected out as a result of an explosion.

But such a scenario raises other questions. Would opposition groups have the ability to manufacture a nerve agent? It would not be impossible, but it is perhaps unlikely given their current strategic priorities and position. Besides, Syria is bound by the laws of war – and if they knew of such a facility, why would they target it, risking a leak and threatening the local population?

There are a number of huge political and diplomatic challenges facing all sides. Priority should be given to establishing a mandate to get international investigators into the location to collect evidence. This should include environmental samples from the ground and unexploded munition carcasses, biological samples from victims as well as witness statements from victims and medical staff. Such samples, properly secured and transported, can then be sent to internationally accredited laboratories where a definitive analysis and identification process can take place.

The importance of a UN/OPCW-mandated investigation cannot be overstated. Individuals, groups or the media may well follow similar protocols in carrying out tests – but they will simply not stand up in the international arena. In the blurring of lines between international politics and investigation best practice, it is vital that there are no doubts that can be seized upon by those who may have something to hide.

Identification of the causal agent will be the first step in establishing the perpetrator. Allowing investigators free access is the best way to ensure that scientific precision, investigative rigour and international transparency trump political machinations.