A team from the Organization for the Prohibition of Chemical Weapons arrived in the Syrian capital, Damascus, on Saturday, with a mandate to investigate the attack. But the team has not yet been able to reach the site.
This has led to accusations by the United States that the Syrian government and Russia are trying to cover up any evidence of chemical weapon use — a charge they deny.
The investigators were originally scheduled to visit the site on Monday but did not. On Tuesday, the Syrian government said the team had not been able to go for security reasons, although it had allowed a number of journalists to visit Douma. Some of those journalists visited the site of the attack and interviewed residents who lost members of their families.
The residents differed on who was responsible for the attack, with some accusing the government and others accusing the rebel group that had been in control of Douma at the time, the Army of Islam.
But the fact that journalists had been able to wander around Douma unmolested raised questions about why it was not deemed safe enough for the investigators to visit.
On Wednesday, Ahmed Uzumcu, the director general of the Organization for the Prohibition of Chemical Weapons, told a meeting of the group’s executive council that a security team from the United Nations had gone to do reconnaissance in Douma on Tuesday.
During a visit to one site, a large crowd gathered and the security team left, Mr. Uzumcu said. At another, the team “came under small-arms fire and an explosive was detonated,” he said.
The team left and returned to Damascus unharmed, but it was not clear when the investigators would be able to visit Douma, Mr. Uzumcu said.
The longer the investigators must wait to take samples in Douma, the lower the likelihood of obtaining evidence of chemical weapons use, because the toxins in the weapons degrade over time.
Determining whether such weapons were used in Douma could help resolve a standoff between the United States and its allies on one side and the Syrian government and its allies on the other.
But under its mandate, the Organization for the Prohibition of Chemical Weapons is only responsible for determining whether chemical weapons were used and what kind, not who used them.
A review by The New York Times of videos of the aftermath and interviews with residents of Douma, medics and rescue workers suggested that metal canisters containing a chemical were dropped from Syrian government helicopters during a wider offensive to rout the rebels.
One of the canisters landed on the upper floor of an apartment building packed with people, many of whom suffocated as the canister’s contents dispersed.