I thought these things didn’t exist.
Update (20150816): But, on the other hand:
While the Sunni Arab terrorists in Iraq have access to people who know how to manufacture mustard and nerve gas, actually doing it is rather difficult. Then, there is the problem of blowback when you do use the stuff for terrorist attacks, and the images of civilians, especially children, injured by these weapons, reach a wide audience. Back in 2007 the chatter among Sunni Arab terrorists, and their supporters, made mention of all this, and apparently no one felt confident enough to “go chemical” in a big way.
Apparently the “chemical option” was not forgotten and someone in ISIL appears eager to revive it. ISIL has not captured any chemical plants capable of manufacturing the deadlier World War I chemical weapons and building such a manufacturing capability from scratch is very difficult and likely to be detected. The chemical threat from ISIL is, however, no longer just theoretical.