I suspect that they will attempt to use some conventional device for some unconventional purpose. An ammonium nitrate style bomb like that used at OKC is much too easy to track and detect nowadays. Likewise, it is very unlikely that Al Qaeda would be able to smuggle in nerve gas shells.What is so hard about smuggling in nerve gas shells? They are compact and produce no telltale radiation.

I think that our seaports are probably most vulnerable. What I worry about most are small nuclear devices that could be stowed away on a large ship. What? Anybody who could not smuggle in a nerve gas shell could certainly not obtain and smuggle a nuke.

Another huge vulnerability are gas pipelines or railroad tank cars routed through urban areas. If you have ever witnessed a propane bleve, you'd know why. I am with you on this one. There are lots of hazardous cargos plying our roadways waiting to be hijacked by Al Qaeda. Fortunately, most are driven by Teamsters who are armed and dangerous.