U.N. human rights investigators have gathered testimony from casualties of Syria’s civil war and medical staff indicating that rebel forces have used the nerve agent sarin, one of the lead investigators said on Sunday.

The United Nations independent commission of inquiry on Syria has not yet seen evidence of government forces having used chemical weapons, which are banned under international law, said commission member Carla Del Ponte.

“Our investigators have been in neighbouring 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 an interview with Swiss-Italian television.

A total of three possible chemical attacks have been reported thus far, and several U.S. officials apparently still maintain varying levels of certainty that the Assad regime is responsible for at least one attack. That said, it will be interesting to see how the Obama Administration reacts should incontrovertible proof emerge, particularly given previous declarations that such attacks from the Syrian government would cross a “red line.”