Terrorists don't need weapons of mass destruction. The "weapons" are right there, in the midst of a large number of targets.
A recent TV news magazine (60 Minutes, if I recall correctly) sent a reporter and camera crew to a number of chemical plants located near or in the midst of population centers in the United States to investigate matters of security. In instance after instance, they were able to walk through unguarded gates or doors into these chemical plants, walk right up to large, often clearly labeled storage tanks containing highly toxic chemicals, stand there for several minutes while the reporter talked at the camera, then walk back out again—without being challenged. It would have been an idiot-simple matter for someone to place plastic explosive and a timer on one or a number of these storage tanks and simply walk out. On one occasion, a security guard walked up to them as they were leaving after having been in the area for nearly half an hour and asked them who they were and what they were doing there.
Remember December 1984, the Union Carbide plant in Bhopal, India? 16,000 dead and thousands more still suffering the after-effects.
And don't think that the terrorists don't know all about this.