A report from the watchdog group that monitors the international chemical weapons treaty concluded Thursday that sarin, a deadly nerve agent, led to the deaths of more than 80 civilians in an April attack on Syria’s northwestern Idlib province. The group, known as the Organization for the Prohibition of Chemical Weapons (OPCW), did not draw conclusions as to who was responsible for the atrocity, arguing that it was outside their mandate. The U.S. has already accused the regime of Syrian President Bashar al-Assad of orchestrating the attack, which it called “reprehensible.”

On Friday, the director-general of the OPCW issued a statement arguing that “the perpetrators of this horrific attack must be held accountable for their crimes.” The Syrian government has denied any involvement, claiming to have destroyed its chemical weapons stockpile following a UN mandate in 2013. Meanwhile, the Russian government, which is allied with Assad, argued that the regime “accidentally hit a stockpile of chemical weapons that belonged to rebels or terrorists.” In the past, Assad has been keen to blame attacks on Syrian rebel fighters.

Three days after the chemical attack on April 4, the U.S. launched 59 tomahawk missiles at a Syrian air base, marking the nation’s first military operation against an Arab government since President Obama’s intervention in Libya in 2011. President Trump was reportedly moved to action after viewing images of Syrian children struggling to breathe from what has now been confirmed as exposure to sarin. “No child of God should ever suffer such horror,” Trump said while announcing the missile strike. Assad has since argued that the U.S. fabricated the story of a chemical weapons attack in an attempt to justify their own military action.