Nuclear Terrorism: A Brief Review of Threats and Responses [Updated February 10, 2005] [open pdf - 96KB]
"It would be difficult for terrorists to mount a nuclear attack on a U.S. city, but such an attack is plausible and would have catastrophic consequences, in one scenario killing over a half-million people and causing damage of over $1 trillion. Terrorists or rogue states might acquire a nuclear weapon in several ways. The nations of greatest concern as potential sources of weapons or fissile materials are widely thought to be Russia and Pakistan. Russia has many tactical nuclear weapons, which tend to be lower in yield but more dispersed and apparently less secure than strategic weapons. It also has much highly enriched uranium (HEU) and weapons-grade plutonium, some said to have inadequate security. Many experts believe that technically sophisticated terrorists could, without state support, fabricate a nuclear bomb from HEU; opinion is divided on whether terrorists could make a bomb using plutonium. The fear regarding Pakistan is that some members of the armed forces might covertly give a weapon to terrorists or that, if President Musharraf were overthrown, an Islamic fundamentalist government or a state of chaos in Pakistan might enable terrorists to obtain a weapon. Terrorists might also obtain HEU from the more than 130 research reactors worldwide that use HEU as fuel."
CRS Report for Congress, RL32595