Chlorine may have been used in the chemical attack that targeted the Syrian town of Douma in April, said the Organization for the Prohibition of Chemical Weapons(OPCW) on Friday.
A team of inspectors from the OPCW took more than 100 samples from some seven sites in Douma when they gained access to the town several weeks after the April 7 attack that left dozens of people dead.
The Syrian regime has been widely blamed for the attack.
"The results show that no organophosphorous nerve agents or their degradation products were detected," the OPCW said in an interim report late Friday.
But it added that the fact-finding mission also found "along with explosive residues, various chlorinated organic chemicals".
It is understood that could mean some samples contained what may be potential markers of exposure to an active source of chlorine, not found naturally in the environment.
"Work by the team to establish the significance of these results is ongoing," the OPCW added.
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 team's mission to Douma was launched amid international outrage over images of adults and children appearing to be suffering from the effects of a toxic weapon attack.
The gruesome footage from the attack horrified the world and prompted unprecedented Western strikes on Syrian regime installations.
The fact-finding mission gained access to Douma on April 21 after several delays after deploying to Syria on April 14.
Last week, world powers voted to grant the OPCW the authority to assign blame in chemical attacks in Syria.
According to the text, the OPCW's secretariat "shall put in place arrangements to identify the perpetrators of the use of chemical weapons in the Syrian Arab Republic."
OPCW director general Ahmet Uzumcu and his successor, who takes over in July, were also mandated to draw up proposals to give the body broader powers to identify those unleashing chemical weapons in any other country, if governments ask for help.