Published on Friday, the preliminary report by the Organization for the Prohibition of Chemical Weapons (OPCW) is based on eyewitness accounts and evidence collected at the site of the attack.
It said chlorine traces were found in two of four investigated locations. No evidence of nerve agents was found.
"Various chlorinated organic chemicals were found in samples from Locations 2 and 4, along with residues of explosive," the OPCW said.
The two locations where no traces were found, a warehouse and a facility, were suspected of being used for the production of chemical weapons.
"From the information gathered during the two on-site visits to these locations, there was no indication of either facility being involved in the production of chemical warfare agents or toxic chemicals for use as weapons," the report said.
It added, however, that it was too early to come to full conclusions, saying that "work by the team to establish the significance of these results is on-going".
Medics and rescuers say about 40 people were killed in the attack, many of them in a housing block hit when a cylinder landed on its roof.
The fact-finding team was still working on "the location of the cylinder, (and) its provenance" which will require a "comprehensive analysis" by experts, the OPCW said.
The town of Douma was under rebel control and facing a government air and ground assault when the suspected attack took place on April 7. Opposition groups gave up the town in the days after the incident.
The OPCW's fact-finding mission was launched amid international outrage over images of adults and children appearing to be suffering from the effects of a toxic weapon attack.
The footage from the alleged attack prompted the United States, Britain and France to launch air raids against military installations in Syria.
Many governments also denounced the alleged chemical attack, blaming it on the Syrian government. Damascus denied the accusations, dismissing them as "farcical".
Russia stuck by its ally Syria and insisted the attack was staged by the White Helmets, a volunteer rescue service operating in rebel-held parts of Syria.
The OPCW does not designate blame for chemical attacks.