There is evidence perpetrators used chlorine in a suspected chemical attack that killed dozens in the Syrian city of Douma in April, according to an interim report from an global chemical weapons watchdog.
"The results show that no organophosphorous nerve agents or their degradation products were detected either in the environmental samples or in the plasma samples from the alleged casualties", the OPCW said.
Nevertheless, "various chlorinated organic chemicals were found in samples from two sites", the report said.
Medics and rescuers say many of those killed died when a cylinder landed on the roof on a housing block.
The gruesome footage from the attack horrified the world and prompted unprecedented Western strikes on Syrian regime installations.
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.
An OPCW fact-finding mission collected samples at the site of the alleged chemical attack in Douma near Damascus.
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Weapons inspectors have concluded there were "chlorinated chemicals" in an attack on a Syrian rebel holdout that triggered retaliatory airstrikes from Britain, France and the US.
The OPCW said that its team was continuing its work to establish the significance of the results."The organization should move quickly to finish its investigation and determine whether a chemical weapon attack occurred" in Douma, New York-based Human Rights Watch said in a statement.
Russian Federation and Syria have sought to disprove that Douma was hit by a poison gas attack.
Daraa is seen as the cradle of the uprising that sparked Syria's seven-year war, and the government retaking full control of it would be a symbolic victory for President Bashar Al-Assad.
Friday's report came a week after the OPCW's top policy-making body agreed that the organisation should have new powers to say who was responsible for any toxic arms attacks in Syria.
"On the basis of the information received and analysed, the prevailing narrative of the interviews, and the results of the laboratory analyses, the FFM can not confidently determine whether or not a specific chemical was used as a weapon in the incidents that took place in the neighbourhood of Al-Hamadaniyah and in the area of Karm al-Tarrab", said the OPCW.