Chemical weapons experts finally collected samples in the besieged Syrian city of Douma on Saturday, the Organisation for the Prohibition of Chemical Weapons said, after waiting for days to access the site of a suspected gas attack.
A fact-finding team visited one site in Douma and collected samples, the group said, to help determine whether banned substances were used there in an April 7 strike.
"The OPCW will evaluate the situation and consider future steps including another possible visit to Douma," the organization said.
The team faced several obstacles in getting to Douma, raising concerns that any chemicals that may have been used could disintegrate by the time experts get there, or that evidence could be manipulated.
UK officials say about 75 people were killed April 7 in an attack on the rebel stronghold. US officials have said they believe chlorine and sarin gas were dropped there.
Both Syria and its most powerful ally, Russia, deny a chemical attack took place.
The United States, the United Kingdom and France have blamed Syrian President Bashar al-Assad's regime for the assault and together carried out airstrikes last weekend in response.
Russia has claimed the April 7 attack was "faked," or even "staged" with the help of British intelligence agencies. Britain denies the allegation.
The OPCW chemical experts arrived last week in Syria's capital, Damascus, but were unable to access a site in Douma until Saturday.
The UK's envoy to the chemical weapons watchdog said Monday that Syria and Russia had blocked access to the experts, which both countries denied.
The US envoy to the OPCW, Kenneth Ward, said Monday he was concerned Russia may have "tampered with" the site of the Douma attack. On the same day, Russian Foreign Minister Sergey Lavrov told the BBC's "HARDtalk" that he guaranteed there had been no Russian tampering, adding his country had sent experts to the site and found "no trace" of chemical weapons.