The nerve agent sarin was used in a chemical attack in April that killed scores of civilians in northern Syria, a global watchdog confirmed Friday, days after the White House accused President Bashar al-Assad’s government of planning a similar strike.

In a statement released ahead of a more thorough fact-finding report, the Organization for the Prohibition of Chemical Weapons (OPCW) described the daybreak assault on the opposition-held town of Khan Sheikhoun as an “atrocity.”

The OPCW’s findings first leaked Thursday when Nikki Haley, the U.S. ambassador to the United Nations, said that the watchdog had confirmed the use of sarin, heightening growing concerns over the Assad government’s chemical stockpile, three years after it was supposed to have been destroyed under the scrutiny of international inspectors.

Entire families were killed in the April 4 attack, some while they were sleeping. A nearby hospital was bombed hours later, forcing rescue workers to load convulsing casualties into pickup trucks and drive them to the next nearest clinic, a half-hour away.