Sarin nerve agent or a similar poison was used in an April 4 aerial attack in northern Syria that killed nearly 100 villagers, including children, the monitoring group that polices the chemical arms ban treaty concluded Thursday in a report shared with United Nations diplomats.

The report by the group, the Organization for the Prohibition of Chemical Weapons, said the conclusions were based on evidence that included analysis of victims, environmental samples and witness interviews.

It did not answer the question of who was responsible for the attack in the rebel-held village of Khan Sheikhoun, regarded as one of the worst atrocities of the six-year-old Syria conflict, where chemical arms have been used multiple times.

While the organization did not make the report public, some diplomats with access to it conveyed the findings. The United States ambassador to the United Nations, Nikki R. Haley, even issued her own news release announcing the report’s basic conclusions.