evidence and match it visually with what we're seeing in that videotape, but how do you determine who was responsible when one side said it was the rebels and the other side says it was sittthe syrian governmen? how do you make that determination? >> there are some characteristics used by sarins used by the syrians. the syrians have what is called a binary form of sarin, two innocuous liquids that when put together form sarin. the chemicals stabilize them and make them less likely to lose their potency, their lethality, quickly. the one the syrians have used is one initially developed by the soviet union. the means were passed to the syrians in the 1970s. you can detect that by analysis. it's highly unlikely that a group of terrorists, in fact, there is no known case that i know of, of a group of terrorists ever stumbling on