Better-than-even odds of a WMD attack?

The odds that terrorists will soon strike a major city with weapons of mass destruction are now better than even, a bipartisan congressionally mandated task force concludes in a draft study that warns of growing threats from rogue states, nuclear smuggling networks and the spread of atomic know-how in the developing world.

That's a scary, attention-grabbing first sentence. And if terrorists ever do, God forbid, pull off a nuclear detonation a major city, people will no doubt point to this commission as prescient.

But let's think about this for a minute. How could they possibly come up with these "odds" of such an event? I'd like to see the methodology.