After several days of delay, chemical weapons experts entered Syria's Douma on Saturday to investigate an alleged deadly chemical attack there on April 7.
Medics and rescuers have said more than 40 people were killed in the then rebel-held town, while Damascus and its ally Moscow have dismissed the accusations as "fabrications".
Two weeks after the alleged chlorine and sarin attack, and amid warnings Syria's regime may have attempted to tamper with evidence, what clues might the team be looking for?
- Chemical traces -
Despite the long delay, inspectors of the Organisation for the Prohibition of Chemical Weapons (OPCW) could still be able to find chemical traces useful for the probe, experts have told AFP.
Evidence could be found in the bodies of alleged victims or in the environment near the site of the suspected attack.
"Autopsy samples, if available, will provide invaluable evidence," said Alastair Hay, professor of environmental toxicology at the University of Leeds.
And "nerve agents like sarin can be present in the environment for many weeks after use and particularly if you look near the site where a weapon has exploded," he told AFP.
Ralf Trapp, a consultant and a previous member of an OPCW mission to Syria, said chlorine gas would be harder to find as it would largely have dissipated into the air.
Instead, as chlorine already exists in the environment, investigators might look for unusually high concentrations of the element in materials from the alleged attack site, he told AFP.
- Tampering? -
Since April 7, thousands of rebels and civilians have been transferred out of Douma on buses to the north of Syria, and Russian and regime security forces have entered the reconquered town.
A member of the White Helmets, civil defence volunteers who reported the attack along with local medics, said they feared the regime had tampered with the burial site of the victims.
But experts said investigators could detect if someone had tried to hide clues.
"You can erase evidence but you need to be thorough. And in being thorough you may well provide clear evidence that the site has been tampered with," Hay said.
Trapp said "cleaning up a contaminated site would involve the removal of materials that cannot easily be decontaminated".
This "would leave signs that things have been removed, in particular if there is video evidence from the time of the alleged attack" for a comparison, he told AFP.
Even after extensive cleaning, "there is a good chance that deposits of the agents or of their degradation products" would remain, he said.
These could be found in materials such as brickwork, concrete and soil, he said.
- What conclusions? -
The team "will bring back all the relevant evidence its inspectors find, including of any clean-up or manipulation" at the site of the alleged attack, Trapp said.
The samples will then be sent off to approved independent laboratories around the world. Each sample is split and sent to at least two different labs.
"If the mission cannot demonstrate that chlorine or sarin were used as weapons at the site, they would present the evidence they have and point to possible scenarios that might explain" it, he said.
It would then "be up to the political organs of the OPCW and to individual states parties to decide whether the evidence was sufficient to confirm a chemical weapons attack or not," he said.
The OPCW is mandated with determining whether or not a chemical weapon was used, but not with deciding who might have carried out such an attack.