US intelligence intercepted communications between Syrian military and chemical experts - CNNPolitics.com
▻http://edition.cnn.com/2017/04/12/politics/us-intelligence-syrian-chemical-weapons
The US did not know prior to the attack it was going to happen, the official emphasized. The US scoops up such a large volume of communications intercepts in areas like Syria and Iraq, the material often is not processed unless there is a particular event that requires analysts to go back and look for supporting intelligence material.
So far there are no intelligence intercepts that have been found directly confirming that Russian military or intelligence officials communicated about the attack. The official said the likelihood is the Russians are more careful in their communications to avoid being intercepted.
The Russian and Syrian governments have both denied involvement in the chemical attack.