British journalist Robert Fisk, writing in The Independent, says when he visited the site in Douma his questions about the attack were met with "genuine perplexity".
This should allow the fact-finding mission to deploy to those locations "at the earliest possible time and based on lessons learned during the advance security visit".
Syria's ally Russian Federation was given prior notice and the missiles struck mostly empty buildings, in what many analysts saw as a hollow move that allowed all sides to save face.
The inspectors arrived in Damascus at the weekend.
The UN team left the site "under the cover and protection of the Russian military police and safely returned to Damascus", it added.
Instead, a United Nations security team entered Douma to "conduct a reconnaissance visit to the sites" ahead of the visit by the inspectors, according to an OPCW statement.
He and two other residents accused the rebel Army of Islam of carrying out the attack.
As with that investigation, the chemical weapons inspectors at Douma will carry out forensic work to find out what happened during the attack.
It was formed after the Chemical Weapons Convention - an arms control treaty that bans the production, stockpiling and use of chemical weapons - entered into force in 1997.
The U.S. -led intervention has threatened to escalate confrontation between the West and Assad's backer Russian Federation, although it has had no impact on the fighting on the ground, in which pro-government forces have pressed on with a campaign to crush the rebellion.
The Syrian state news agency SANA earlier reported that the worldwide experts from the Organisation for the Prohibition of Chemical Weapons (OPCW) had entered Douma to begin their investigation of whether chemical agents were used as a weapon. "There is still a lot of volatility in the area", United Nations spokesman Stephane Dujarric said, adding that the United Nations security team needed to make at least another visit before the fact-finding mission could go ahead. Britain's ambassador to the OPCW Peter Wilson said it was now unclear when the inspectors would be able to reach it.
Global chemical weapons inspectors do not appear to have visited the site of a suspected attack in Syria after days of delays by Syrian and Russian authorities.
Security Council members have been trying to set up a new inquiry mechanism for chemical attacks in Syria since such a mechanism was disbanded in November 2017 after Russian Federation vetoed the extension of its mandate. Russia's Foreign Ministry slammed this allegation as a bogus story, while the country's Defense Ministry pointed out that the White Helmets was not a reliable source of information, as they were known for disseminating fake news.