Investigators from the global chemical arms watchdog collected samples Saturday from the site of a suspected gas attack in Syria two weeks ago, after security concerns delayed their probe.
Gruesome footage of the alleged toxic attack on April 7 in the town of Douma, just outside Damascus, horrified the world and prompted unprecedented Western strikes on Syrian military installations.
Just hours after the joint missile strikes, a team from the Organisation for the Prohibition of Chemical Weapons (OPCW) arrived in Syria to investigate the claims.
But they had been unable to access Douma itself, which fell from rebel hands into joint Syrian and Russian control after the suspected chemical attack.
United Nations security experts deployed to the town on Tuesday to check it was safe, but were forced to pull out after an explosion went off and they were shot at.
Investigators finally collected evidence from the site for the first time on Saturday, the OPCW said.
"The samples collected will be transported to the OPCW Laboratory in Rijswijk and then dispatched for analysis to the OPCW's designated labs," the body said in a statement, adding it could plan another visit.
Russia's foreign ministry had earlier announced the inspectors reached Douma after guarantees by Syria and its own forces, and said it expected them to carry out an "impartial investigation".
Moscow, a key ally of President Bashar al-Assad, has said its own probe into the events in Douma found no traces of chemical use.
Assad's government, too, has repeatedly denied using toxic weapons, and accused the West of "fabricating" the claim to justify bombing Syria.
Medics and first responders in the town say more than 40 people were killed on the night of April 7, reporting foaming at the mouth, pinpoint pupils and difficulty breathing.
Investigators reached Syria a week later at the country's invitation, arriving just hours after a coordinated wave of French, American and British missile strikes.
But they spent a week in the capital, unable to access Douma just a few miles to the east, even as groups of journalists visited the town on government tours.
Western powers slammed Syrian and Russian authorities, saying proof that chemicals were used in Douma was probably being hidden.
"It seems likely that this attitude is intended to make proof and material evidence linked to the chemical attack disappear," Foreign Minister Jean-Yves Le Drian said on Saturday, shortly before the OPCW got access to Douma.
Analysts say that if toxic chemicals were used, OPCW inspectors would still be able to find important traces in clothes, walls, rocks and soil in Douma, although their potency decreases with time.
"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," said Alastair Hay, professor of environmental toxicology at the University of Leeds.
Crucial evidence can also linger in victims' blood, urine and organs.
"Autopsy samples, if available, will provide invaluable evidence and nerve agents can be found in many organs," Hay told AFP.
The "White Helmets", an alleged Syrian rescue force that works in opposition-held areas, said it had given the OPCW details on the burial site of the Douma victims.
But a top official from Jaish al-Islam, the rebel group that controlled Douma during the alleged chemical attack, accused the regime of raiding the graves.
The OPCW does not have a mandate to establish who is responsible for chemical attacks. The joint OPCW-UN taskforce that did was shut down by Russia last year after it blamed Syria's regime for another gas attack.
Experts say it may take between two and three weeks to analyse evidence collected in Douma.
The town was the last in the Eastern Ghouta suburb to fall to Syria's regime, after a blistering two-month offensive backed by Russia.
Since securing Ghouta, Syrian troops have shifted their focus to a pocket of territory in southern Damascus still held by the Islamic State group.
Heavy air strikes pounded the Yarmuk Palestinian camp and adjacent Hajar al-Aswad district on Saturday.
Syria's conflict has killed 350,000 people and defied UN peacemaking efforts since it erupted in 2011 with protests against Assad.
On Saturday, the 15 UN Security Council ambassadors and UN Secretary-General Antonio Guterres were huddling in Sweden, a non-permanent member of the body, to try to hash out a solution.
It was a first for the Council, which normally holds its annual brainstorming session in upstate New York.