A just-released report (warning: graphic cover page) from Human Rights Watch, based on evidence and interviews with those in the area, concludes that the chemical attacks carried out on August 21 were the action of the Syrian government and not rebel forces.
Though the report's authors could not actually investigate the Ghouta area where the rockets landed, they interviewed more than 10 eyewitnesses and three responding doctors via Skype to verify the use of chemical weapons and the exact types of rockets used. The rockets used in the attacks—330mm and 140mm surface-to-surface rockets—could only have been possessed by Assad's forces:
The evidence concerning the type of rockets and launchers used in these attacks strongly suggests that these are weapon systems known and documented to be only in the possession of, and used by, Syrian government armed forces. Human Rights Watch and arms experts monitoring the use of weaponry in Syria have not documented Syrian opposition forces to be in the possession of the 140mm and 330mm rockets used in the attack, or their associated launchers.
The doctors interviewed said that the symptoms correlated with Sarin gas or some other nerve agent.
The report also refutes claims that the attacks were caused by Syrian opposition forces mishandling chemical agents, noting "Claims that the August 21 deaths were caused by an accidental explosion by opposition forces mishandling chemical weapons in their possession are inconsistent with large numbers of deaths at two locations 16 kilometers apart."
On Monday, President Obama said that he had discussed with Vladimir Putin the possibility of placing Syria's chemical weapons under international control.