The Hague (AFP) - Scientists, paramedics and experts from the OPCW chemical watchdog are in Syria to investigate allegations of last weekend's poison gas attack.
After last month's probe in the British town of Salisbury, it is the second time in just a few weeks that investigators from the Organisation for the Prohibition of Chemical Weapons (OPCW) have been deployed to a site of a suspected toxic arms attack.
Here are a few questions and answers about the experts' work behind the scenes.
- How is an OPCW team deployed? -
Normal preparation time for a fact-finding mission takes between two to four weeks.
But the OPCW's chief Ahmet Uzumcu told AFP in 2016 that amid increasing allegations of chemical weapons use in Syria he had set up a special team of up to 15 people who deploy within 24 hours.
Once a mission is agreed upon, the equipment store team at the OPCW's facilities in Rijswijk gather and check a wealth of instruments and gear needed to protect the experts and aid their analysis.
While inspectors want to travel as light as possible, they also need to ensure they carry the necessary equipment.
This can vary from air cylinders, to sample vials for gathering such things as soil, blood, or plants from the area of an alleged attack.
Chemical vapour detectors, sample scrapers and detection papers for testing water, are also all packed into hard-shell plastic cases for safe transport.
More sophisticated equipment includes detectors using flame photometry, in which an air sample is burned in a hydrogen-rich flame commonly used for detecting gas.
Each team member is fitted with gas masks, or industrial respirators, as well as hazmat protection suits or air-tight decontamination suits.
Satellite phones and emergency medical kits equipped with various antidotes are also essential.
- Who are the team members? -
A typical team could be composed of two medical doctors, two chemical weapons munitions specialist inspectors, one or two analytical chemicals inspectors, interpreters and security.
They can be of all different nationalities and are experts in their field, who will have undergone extensive training.
Once at the site, the team seeks to carry out interviews with witnesses, doctors and first responders as well as review any hospital records.
Apart from environmental samples, they will also collect biomedical samples, sometimes from those killed in an incident, but more preferably from survivors.
- Difficulties in a war zone? -
OPCW experts had never deployed into a war zone before they were called on in 2013 to help the United Nations probe allegations of chemical weapons use in Syria.
This is the first OPCW mission to deploy outside of Damascus since 2014 when a team was ambushed with a roadside bomb and then came under attack, before two were briefly detained.
Both Syria and its ally Russia have said they will ensure the Douma mission's safety and security.
But the nature of the conflict means teams have tight deadlines to work under, in constantly evolving situations. They have been working closely with UN security staff to evaluate the situation on the ground.
In the 2013 mission which confirmed that sarin gas had been used in an August attack which killed hundreds of people outside Damascus, the team only had about 45 minutes to collect samples before pulling out for safety concerns.
- What happens next? -
Once the samples are taken from the site they are brought back to The Netherlands for analysis.
"Our inspectors take the sample, they witness it, they log it, they seal the sample, they keep track of the sample all the time until the analysis is done," former head of the OPCW lab Hugh Gregg said in a 2016 interview with an online news portal.
"So a complete 100 percent chain of custody is absolutely critical."
The samples are split back in Rijswijk, on the outskirts of The Hague, and sent to partner laboratories for further analysis.
The watchdog has about over a dozen labs around the world which it works with, described as "the lynchpin of the OPCW's verification regime" but which are "bound by secrecy agreements."
They offer the "necessary assurance" for member states that chemical analyses are "carried out competently, impartially and with unambiguous results," a spokesperson told AFP.
Source: The OPCW's website, its documents and YouTube channel.