The Organisation for the Prohibition of Chemical Weapons (OPCW) showed Reuters a report that claims the Islamic State (ISIS/ISIL/IS) and another rebel group in Syria used chemical weapons.

The report concluded that “at least two people were exposed to sulphur mustard” in Marea on August 21.

“It is very likely that the effects of sulphur mustard resulted in the death of a baby,” read the report.

It is the first time a report has claimed rebels used sulfur mustard, also known as mustard gas, during the civil war. The Syrian regime promised to destroy their chemical weapons and allegedly handed over their “toxic chemicals 18 months ago.”

“It raises the major question of where the sulphur mustard came from,” a source told Reuters. “Either they (IS) gained the ability to make it themselves, or it may have come from an undeclared stockpile overtaken by IS. Both are worrying options.”

Using any chemical weapons “violates U.N. Security Council resolutions and the 1997 Chemical Weapons Convention.”

A rival Syrian rebel commander made these claims against ISIS after the attack. Medical charity group Médicins Sans Frontières and the Syrian American Medical Association also accused ISIS of using mustard gas against the civilians in Marea.

This is not the first accusation against ISIS concerning chemical weapons. The media reported in June 2014, when ISIS expanded its Caliphate into Iraq, the terrorist group captured weapons that belonged to dictator Saddam Hussein.

A month later, the Kurdish Peshmerga told Kurdish media outlets that ISIS used these weapons in Syrian Kurdistan:

Kobanê canton has been exposed to the cruel attacks of the terrorist ISIS militants for some time. In these attacks, ISIS gangs are using all kinds of weapons, including the thermal missiles of USA. Nonetheless, after the first researches and medical control which was done by health team of Kobanê canton and experts on the wounded and martyred fighters, it has been proved that the ISIS gangs have used chemical weapons. Doctors found burns and white dots on the bodies of the martyrs.

These forces made the same claims in March. The Kurdish Regional Security Council said ISIS attacked the Kurds in January when they drove a truck full of chemical weapons past the troops on a road near the border of Syria. When the Kurds hit the truck, it released gas into the air, causing soldiers to suffer from “dizziness, nausea, vomiting, and general weakness.” This led officials to conclude ISIS used chlorine gas.

In July, two U.K. organizations reported that ISIS used chemical weapons in Iraq and Syria. A mortar at the Mosul Dam was still leaking a dark yellow liquid after the attack. Those who came close to the liquid suffered similar symptoms. The groups also looked at weapons used in an attack on Tel Brak and Hasakah.

“Nine days later, when researchers examined fragments of munitions at Tel Brak, they were covered in a chemical residue which still had an acrid odor and caused powerful throat and eye irritation,” stated CNN. “At a hospital in Qamishli, several of the affected fighters tested positive for PH3, a phosphine-based chemical used as an insecticide or fumigating compound.”