THE HAGUE, Netherlands -- An investigation by the international chemical weapons watchdog has concluded that sarin or a sarin-like substance was used as a chemical weapon in an April 4 attack on a Syrian town that left more than 90 people dead, diplomats said Thursday.

The report by the Organization for the Prohibition of Chemical Weapons into the Khan Sheikhoun attack was not publicly released, but two diplomats who saw its contents confirmed the key finding that sarin was used. One of the diplomats spoke on condition of anonymity because the report had not been made public.

U.S. envoy to the United Nations Nikki Haley issued a written statement expressing confidence in the report, "which confirms what we already knew: chemical weapons were used against the Syrian people."

"Now that we know the undeniable truth, we look forward to an independent investigation to confirm exactly who was responsible for these brutal attacks so we can find justice for the victims," Haley added.

The OPCW investigation did not have a mandate to apportion blame. It will be sent to a joint United Nations-OPCW investigative mission that aims to establish who was responsible for the attack.

The Khan Sheikhoun attack caused an international uproar as photos and video of the aftermath, including quivering children dying on camera, were widely broadcast.

U.S. President Donald Trump cited the images when he launched a punitive strike days later, firing cruise missiles on a Syrian government-controlled air base from where U.S. officials said the Syrian military had launched the chemical attack.

It was the first direct American assault on the Syrian government and Trump's most dramatic military order since becoming president months before.

Syrian President Bashar Assad denied responsibility for the attack. Syrian ally Russia argued after the incident that the victims had died of exposure to toxic agents released when Syrian warplanes hit a rebels' chemical weapons depot.

The conclusion that sarin was used had been expected. The OPCW Director-General, Ahmet Uzumcu, said two weeks after the attack that tests carried out on samples taken from victims and survivors indicated they had been exposed to sarin or a sarin-like substance.

The Syrian government joined the OPCW in 2013 after it was blamed for a deadly poison gas attack in a Damascus suburb. As it joined, Assad's government declared some 1,300 tons of chemical weapons and precursor chemicals which were subsequently destroyed in an unprecedented international operation.

However, the organization still has unanswered questions about the completeness of Syria's initial declaration, meaning that it has never conclusively been able to confirm that the country has no more chemical weapons.

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Associated Press writer Edith M. Lederer at the United Nations contributed.