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The calls are getting louder. Even Russia, Syria’s main ally, is now urging the Syrian government to allow weapons inspectors to investigate the site of Wednesday’s alleged chemical attack on the suburbs of Damascus, held by rebels opposing President Bashar al-Assad’s regime. This brings Russia in line with the US and the 36 other countries that yesterday called for a swift investigation into the apparent deaths of over 1000 men, women and children.

Even if the inspectors – who are already in the country looking into three other unverified attacks – are given permission to investigate the site, there is no guarantee that they will be able to establish conclusively what has happened. But the tension is cranking up. US president, Barack Obama, who a year ago described the use of chemical weapons as a “red line”, .

Right now, the evidence that a chemical attack has taken place is based on , eye-witness accounts and . All point to a chemical weapon attack, but there is the possibility, albeit an unlikely one given the scarcity of reports, that the rocket contained no chemical payload but struck a chemical factory or warehouse.

Sarin suspected

However most experts agree that the videos of the people affected indicate something more sinister. “They provide individual visual evidence of people who are clearly affected by something,” says Howard Hu of the University of Toronto, Canada, a medical advisor for the non-profit group .

Hu says the victims’ rigid muscles and spasmodic movements suggest a neurological organophosphate toxin, such as sarin, but only good close-up images would allow for a more definitive assessment of the symptoms of sarin poisoning, which include drooling and constricted pupils.

at the University of Leeds, UK, who has investigated a number of real and alleged chemical attacks, agrees. “Many of the victims have individual signs suggestive of exposure to an organophosphate agent. Pinpoint pupils are certainly one of the signs but [pupils] should not be used to rule out exposure: when people are afraid their pupils dilate and this may be the initial appearance”.

The victims in the footage also show clear signs of asphyxiation, says , formerly of the European Institute of Security Studies in Paris. “With asphyxiation, you see colouring of the skin – a blue-pinkish hue, and a waxen look,” he says. This is a sign the nervous system is affected and the major organs – for example, the heart and the lungs – are having trouble communicating with each other.

Victims hold clues

According to the , the Syrian government is believed to have stockpiles of the nerve agents VX and sarin, as well as the blister agent, mustard gas.

If they can get to the scene, UN inspectors could gather more decisive evidence, but the Syrian government has so far been reluctant to grant full access to the site. The amount of time that any evidence will remain there for will depend on what the agent is. In a temperate climate, sarin evaporates at a similar rate to water, says Zanders, but the processes by which it breaks down, and the metabolites it produces in the body, are well known. This means it should be possible to detect even if the inspectors don’t get in immediately. “Some things can be traced many months after the incident,” Zanders says.

If the inspectors do gain access, the first order of business will be to collect physiological samples from the living victims and the bodies of dead ones. These could include tissue, blood, urine, hair and skin samples. If they were able, full post-mortems of the victims would be useful. The inspectors could also collect environmental samples, like leaves, soil or wallpaper and the corpses of any affected animals.

Covert agents

In the past, weapons inspectors have used portable equipment to run gas chromatography and mass spectrometry on the samples, which can separate the samples into their physical components and analyse them.

If this team doesn’t have that equipment in the field, they would have to freeze the biological samples and ship them to a lab certified by the Organisation for the Prohibition of Chemical Weapons.

There’s always the chance that samples will be smuggled out of Syria for analysis before the inspectors gain access. President Obama has instructed the US intelligence community to gather information about the recent attack. This may involve using covert agents to gather samples and smuggle them out of the country, as they have done in past conflicts.

Of course – the most conclusive evidence would be an analysis of the remains of the munition used to deliver the weapon. This could prove that it was a malicious attack, and what chemical was deployed.

Fact collectors

But even this wouldn’t necessarily shed any light on who was behind the attack, and this is not part of the inspectors’ mandate. “The UN secretary general has been careful to say that it’s not their job to attribute, it’s their job to investigate, to collect facts,” says of the Center for Nonproliferation Studies in Washington DC.

Pictures of the rocket used in the latest attack show the sort of homemade weapon associated with previous small-scale attacks in the country that some link to rebel groups. After a chemical attack on 19 March 2012 in Khan al-Assal in Aleppo, a Russian team were allowed to carry out tests. The Russian ambassador to the UN told a press conference that both the , and therefore most likely to have been fired by a government opposition group.

However, in June this year, the US government said it was confident that it was Assad’s forces that had carried out the Khan al-Assal attack, as well as several others over the previous year.

Today the UK foreign minister . “I know that some people in the world would like to say that this is some kind of conspiracy brought about by the opposition in Syria,” he said in an interview. “I think the chances of that are vanishingly small and so we do believe that this is a chemical attack by the Assad regime.”

So even if scientific inspection is possible, different sides will probably draw very different conclusions.