Inspectors have conclusively identified mustard gas as the toxic agent used in an attack by insurgents in northern Syria this summer, according to a statement released Friday by an international chemical weapons watchdog.

The findings by the Organization for the Prohibition of Chemical Weapons (OPCW) are the first confirmation that non-state actors have used mustard gas in Syria’s four-year-old conflict. They also are the first confirmation of the use of the toxin since the Syrian government agreed two years ago to destroy its stockpile of chemical weapons. Since then, Syrian President Bashar al-Assad’s forces have been accused of carrying out many attacks using chlorine gas, a choking agent.

The OPCW did not specify which group used mustard gas in the attack, which the organization said killed at least one person, an infant, on Aug. 21 in the village of Marea. Doctors and an aid organization in the village north of Aleppo blamed the attack on Islamic State militants who for months have been battling rebel groups in the area.

The use of mustard gas by either the Islamic State or rebels, or both, is a significant escalation in a conflict that has killed 250,000 people and displaced millions.

“In this case, the team was able to confirm with utmost confidence that at least two people were exposed to sulfur mustard, and that it is very likely that the effects of this chemical weapon resulted in the death of an infant,” the OPCW statement said.

Sy Hersh wrote about this over a year ago, noting that Turkey has been actively supporting chemical weapons development by ISIS and other Islamic insurgents in Syria.

The manufacture of mustard gas is not that difficult, particularly with state support.