I'm going to try to start.
I can't speak to the science behind the conversion, but I think there is a broader point of the ongoing interest of certain terrorist groups in weapons of mass destruction. Previously al Qaeda has clearly indicated an interest in obtaining anthrax and in obtaining improvised nuclear weapons. The technical hurdles to doing that are still very high, but I don't think you can discount the effects of any kind of an attempt, even a small initiative by al Qaeda or any terrorist groups using weapons of mass destruction—not necessarily just the physical harm, but you can think of the economic repercussions, the psychological effect. I think it's not just the scientific ability to go up the knowledge curve to be able to pull off a nuclear attack; it's the aspirational attempt, which is still a big part of the threat assessment.