Western intelligence agencies say they are convinced beyond reasonable doubt that President Bashar al-Assad's government carried out the worst chemical weapons attack for decades.
A new report on the gas attack in Syria last month says the amount of toxic material used was bigger than first thought.
Researchers in the US say a study of photos from the incident seem to show rockets carried fifty times more nerve agent.
Al Jazeera's James Bays reports from Massachusetts.
Source: Al Jazeera