Why they are convinced Russia is lying:

“Level of cooperation” between Russia and Syria and holes in Russian narrative make it “clear that the Russians are trying to cover up what happened here.”

Russia says the deadly gas was released after a bomb hit a munitions depot, but the munition the US has seen containing sarin landed in the middle of the street.

“We do think that it’s a question worth asking the Russians, about how is it possible that their forces were co-located with the Syrian forces that prepared and carried out the chemical weapons attack and did not have foreknowledge.”