There are more and more reports coming out of the Middle East of both Syria and IS using improvised chemical munitions. These devices have generally used industrial chemicals (typically chlorine gas) rather than military grade chemical munitions (chlorine was used as the original chemical weapon, but it has long ceased to be considered an effective military grade chemical weapon). There is a brief summary article here that hits some of the high points and recent a New York Times article shows how easy these weapons are to make.
While there is a VERY outside possibility that some of these actual weapons will be sent to the United States, the much higher concern is that the development of very rudimentary weapons point to the fact that they could very easily be made in the United States by anyone with a moderately equipped home workshop. It would be very unusual for IS not to export the knowledge of how to make these weapons.
These are very definitely tactical scale terror weapons. While they do fit the current legal standard for being weapons of mass destruction (WMD) they should more probably be classified as weapons of mass hysteria (WMH). The concentration of these industrial gasses necessary to kill is relatively high and hard to achieve over any substantial area with these types of weapons. A limited number of people could certainly be killed by these weapons, but the hysteria resulting from their use would be much more devastating in both the long run and short run than would be the actual injuries from the chemicals.
It really is not hard to get your hands on some of the chemicals of interest that would be used in these devices. The Chemical Facility Anti-Terrorism Standards (CFATS) program does regulate the security at some facilities that manufacture, store or use, but not all of them by a long shot. Small water treatment facilities are exempt from both CFATS and EPA security programs and they frequently use small cylinders of chlorine gas. Small agricultural users of anhydrous ammonia are not currently under any security regulations.
I suspect that we are going to be hearing more about these types of attacks outside of the current conflict zone in the not too distant future.