The Organisation for the Prohibition of Chemical Weapons (OPCW) reported that the toxic chemical used in Douma, was likely molecular chlorine.
Despite being given the power to assign blame by its member states last June, the OPCW investigation did not do so for the Douma attack, as it was not part of the mandate for the team investigating the April's attack.
The last rebel-held suburb of Damascus was being beseiged by Syrian forces in April 2018. Dozens of civilians were killed in the attack.
The OPCW fact-finding mission visited the town last April to collect environmental samples, collect data and interview witnesses.
A joint UN-OPCW team was appointed to assign blame for chemical attacks in Syria. It found that Syrian government troops had used the nerve agent sarin and chorine barrel bombs on several occasions, while Islamic State militants were found to have used sulphur mustard.
The OPCW said on its website Friday that its "Technical Secretariat is putting in place arrangements to identify the perpetrators of the use of chemical weapons in the Syrian Arab Republic."
It is also looking into an alleged gas attack last November in Aleppo that reportedly made up to 100 people ill.
The Syrian regime has been accused by Western powers of using chemical weapons on dozens of occasions, including in Khan Sheikhoun, Latamenah and Saraqeb.