John Mueller, a political science professor at Ohio State University, argues that there is an "almost vanishingly small" likelihood that terrorists would ever be able to acquire and detonate a nuclear weapon. In even the most likely scenario of nuclear terrorism, there are 20 barriers between extremists and a successful nuclear strike on a major city, according to Mueller.

Even as the United States spends billions of dollars on biological defense initiatives, experts continue to debate the likelihood that terrorists could pull off a major attack using smallpox or another disease agent.

Michael Krepon, a longtime expert on nuclear threat reduction, is questioning a finding released this week asserting the likelihood that a weapon of mass destruction will be used somewhere around the globe within the next five years.