It’s hard to say - could have been either. But it’s interesting that Assad shelled the area a day or 2 after the story broke....trying to cover up evidence & keep the U.N. out?
May 2013 UN points the finger at the rebels.
http://rt.com/news/un-syria-rebels-chemical-weapons-854/
UN human rights investigators have spoken to the victims of Syria’s civil war and gathered medical testimonies which point to the Syrian rebels having used sarin nerve gas, while any allegations of its use by the government remain unsubstantiated.
The United Nations independent commission of inquiry on Syria has concluded that no evidence of the use of sarin by Syria’s government troops has so far been uncovered, said the lead commission member Carla Del Ponte on Sunday.
In an interview to Swiss-Italian television, Del Ponte revealed that the “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.”
Maybe... But If I had intel that my enemies had chem weapons, I'd shell them to try and hoist them on their own petard.