Nuclear terrorism is a real and dangerous threat. Some terrorist organizations, particularly Al Qaeda, are seeking nuclear weapons. While the overthrow of the Taliban and the disruption of Al Qaeda’s old central command structure certainly reduced Al Qaeda’s chances of pulling off such a complex operation, Al Qaeda is reconstituting in the tribal areas of Pakistan. Making nuclear bomb material from scratch is beyond the plausible capabilities of terrorist groups, but if a sophisticated group got enough highly enriched uranium (HEU) or separated plutonium, it might well be able to make a crude nuclear bomb. Indeed, the U.S. intelligence community assesses that an Al Qaeda nuclear bomb effort probably would not require the involvement of more than the number of operatives who carried out the September 11, 2001 attacks, and could be just as compartmented, making it extraordinarily difficult for the intelligence community to detect and stop.
Bunn, Matthew. "Preventing Nuclear Terrorism." 2009 National Security and Nonproliferation Briefing Book. Peace and Security Initiative, 2008.