A UN investigator on Sunday indicated that Syrian rebels, not the Assad regime, have used chemical weapons in the two-year civil war, the inverse of previous assessments by American and Western authorities.

Carla Del Ponte, head of the independent UN commission investigating reports of chemical weapons use in Syria, told a Swiss-Italian television station that UN investigators gleaned testimony from victims of Syria’s civil war and medical staff which indicated that rebel forces used sarin gas — a deadly nerve agent.

“Our investigators have been in neighboring countries interviewing victims, doctors and field hospitals and, according to their report of last week which I have seen, there are strong, concrete suspicions but not yet incontrovertible proof of the use of sarin gas, from the way the victims were treated,” Del Ponte said in the interview, translated by Reuters.

“This was use on the part of the opposition, the rebels, not by the government authorities,” she added.

Del Ponte was commissioned by the UN Human Rights Council to investigate claims by Syrian opposition groups and the Assad government that either side used chemical weapons in clashes between the two sides. Syria also has asked the UN to investigate a chemical weapons attack in March on the village of Khan al-Assal outside of Aleppo, which the regime blames on rebels and the rebels blame on regime forces. UN Secretary-General Ban Ki-moon commissioned a separate investigation into the claims, but inspectors have not been allowed into the country.

Del Ponte stated in February that both sides of the Syrian civil war had committed atrocities and should be brought to justice.

According to a report in the Daily Beast published Thursday, US military and intelligence officials acknowledged that they don’t know where much of Syria’s sizable chemical weapons stockpile is presently located. No reports from Syria indicated that opposition forces captured any of President Bashar Assad’s chemical weapons arsenal.

US President Barack Obama previously indicated that chemical weapons use by the Assad regime constituted a “red line” for the United States. Following claims by US officials that they had definitive proof that Assad forces used chemical weapons, Obama last week appealed for patience, saying he needed more conclusive evidence about how and when chemical weapons detected by US intelligence agencies were used and who deployed them.

Obama signaled he would consider US military action against Syria if “hard, effective evidence” were found to bolster intelligence that chemical weapons have been used in the two-year-old civil war.