On Thursday US government officials offered “zero real evidence” that the Syrian government was behind the alleged chemical attack in Douma.

US Congressman Thomas Massie announced that the US director of national intelligence (DNI), the defense secretary and the secretary of state had “provided zero evidence” showing Syrian President Bashar al-Assad used chemical weapons against the Syrian people in Douma.

In their briefing to Congress, the director of national intelligence and the secretaries of defense and state referenced info circulating online. Which means they either chose not to provide proof to Congress or they don't have conclusive proof that Assad carried out gas attack.