The daybreak attack, it said, was one of more than 20 government assaults involving chemical weapons since March 2013, most of them targeting families with no part in the conflict.
This is the first time that a United Nations body has explicitly accused the Syrian government of using sarin, a chemical which pushes the nervous system into overdrive and can kill in minutes.
Video footage from the scene of the attack on the northern village of Khan Sheikhoun showed men, women and infants convulsing uncontrollably. In many cases, they had no idea what had hit them - sarin is colorless, odorless and tasteless.
The attack killed at least 83 people, dozens of them women and children. According to investigators, some died in their beds. A single mother who had left the house early to work said that she returned to find all her four children dead.
Images of the youngest casualties are believed to have figured in President Donald Trump's calculation to order missile strikes on a Syrian government airstrip days later, marking the first direct American military intervention against Syrian President Bashar Assad.
The Syrian government and its Russian backers had insisted that the Khan Sheikhoun attack was the fault of opposition forces in the area, or that it was entirely fabricated.
"The Commission considered and investigated all potential scenarios, including claims put forward by Russian and Syrian officials," it said, concluding that it had found no supporting evidence for either claim.
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