The United States is investigating whether Russia was complicit in an alleged chemical weapons attack by the Syrian military on civilians, but has not yet made a determination, a senior administration official said Tuesday.

"How is it possible that their forces were co-located with the Syrian forces that planned prepared and carried out this chemical weapons attack at the same installation and did not have foreknowledge?" said the official, speaking on condition of anonymity.

"We do think it is a question worth asking the Russians." 

"We've seen these militaries operate very closely, even down to an operational level," he said, adding "there is not a consensus on our side about the extent or how to interpret the information we have and continue to get."