A fact-finding mission by the UN’s chemical watchdog, the OPCW, has concluded that sarin or a sarin-like substance was used as a chemical weapon in the April 4 attack in the Syrian town of Khan Sheikhoun.

The confidential report released on Thursday by the Organization for the Prohibition of Chemical Weapons (OPCW) will now be taken up by a joint UN-OPCW panel to determine whether Syrian government forces were behind the attack.

“Based on its work, the FFM (fact-finding mission) is able to conclude that a large number of people, some of whom died, were exposed to sarin or a sarin-like substance,” said the report, parts of which were obtained by AFP news agency.

“The release that caused this exposure was most likely initiated at the site where there is now a crater in the road,” it added.

“It is the conclusion of the FFM that such a release can only be determined as the use of sarin, as a chemical weapon.”

At least 87 people, including many children, were killed in the attack that the United States, France and Britain have said was carried out by President Bashar al-Assad’s forces.

The US launched a retaliatory cruise missile strike days later against a Syrian airbase from where it alleges the chemical weapons attack was launched.

The OPCW did not visit the Shayrat airbase in Homs or the town of Khan Sheikhoun, where the attack took place, in rebel-held Idlib.

US Ambassador Nikki Haley said in a statement that she had the “highest confidence in the OPCW report”.

“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,” she added.

The OPCW-UN joint investigative mechanism (JIM) has already determined that Syrian government forces were responsible for chlorine attacks on three villages in 2014 and 2015, and that the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) used mustard gas in 2015.

“It’s good that the UN agency has come out publicly to say that it was sarin nerve agent that was used,” Hamish de Bretton-Gordon, chemical weapons adviser, told Al Jazeera.

“It’s a great disappointment that they haven’t had the mandate to apportion blame.”

READ MORE: Khan Sheikhoun attack survivors recall horror

The Syrian government 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 tonnes of chemical weapons and precursor chemicals that were subsequently destroyed in an unprecedented international operation.

However, the organisation 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.