Even before investigations could be conducted, - "Assad government bears responsibility for 'chemical' attack." - http://www.informationclearinghouse.info/46801.htm
Lest we forget:
Seymour M. Hersh: Whose sarin?
The U.S. intelligence agencies produced evidence that the al-Nusra Front, a jihadi group affiliated with al-Qaida, had mastered the mechanics of creating sarin and was capable of manufacturing it in quantity.