Syria has accused rebels and emergency workers of staging the attack it by bringing in dead bodies and filming them, or alternatively arguing that an Islamist chemical weapons factory was hit.
But the OPCW said its team "thoroughly pursued lines of inquiry and scenarios suggested by Syrian authorities and other state parties, but was unable to obtain any concrete information supporting them."
It also said it "regrets" that Syria refused to allow it further access to the site to investigate.
The report also dismisses claims by former inspectors who alleged that the watchdog had altered original findings in 2018 to make the evidence of a chemical attack seem more conclusive.