"There can be no doubt about the fact that enough nuclear material to build more than twenty nuclear weapons was lost in the transition from the Soviet Union to Russia," wrote Harvard scholar Graham Allison.

Russian officials appeared to confirm this assertion. In a May 1997 private meeting with members of Congress, General Alexander Lebed, national security advisor to then- Russian President Boris Yeltsin, acknowledged that Moscow could not account for over 100 1-kiloton Soviet suitcase nuclear devices.

Although Moscow attempted to deny the revelation and discredit its source, U.S. government analysts have never succeeded in clarifying the matter.

Following the September 11, 2001 attacks, then-President Vladimir Putin told U.S. President George W. Bush that he could not account for the security of Russian nuclear material during his predecessor's administration.

Despite such high-level admissions of likely nuclear leakage, select analysts have dismissed nuclear smuggling as an inconvenient feature of the existing nonproliferation regime rather than a strategic threat in its own right. They point out that total seizures of highly enriched uranium and plutonium between 1992 and 2006 amounted to less than a pound - insufficient to manufacture a bomb.

Nevertheless, disturbing signs point to the existence of a nuclear black market that, like all markets, "is evolving, and becoming more professional, proving itself surprisingly resilient," as Ron Suskind wrote in "The Way of the World: A Story of Truth and Hope in an Age of Extremism."

In Russia, the end of the Cold War created an environment highly conducive to nuclear leakage: a steep decline in government orders for nuclear goods, weakened security controls, economic hardships among the country's weapons scientists and other nuclear workers. The current global financial crisis with its attendant reduction in Russian defense expenditures may recreate these conditions, despite President Dimitry Medvedev's promise to retain Moscow's present level of investment in the defense sector.

Similarly, pressure is evident on the demand side as well. Intelligence and media reports suggest that several states, such as Iran and possibly North Korea, and sub-national groups, including Al Qaeda in the 1990s, have been - or currently are - interested in obtaining stolen nuclear materials. Nor are the final buyers always apparent. For instance, independent apprehensions of nuclear smugglers on the Russian border with Georgia in 2003 and 2006 seem to suggest that purveyors of nuclear material may be communicating with customers in ways not readily apparent to Western intelligence or law enforcement officials. In both cases, the men claimed to have access to additional nuclear material, besides the samples in their possession. The larger cache has remained unaccounted for, as is typical in such seizures.

Given the enormity of the stakes, it stands to reason that improved intelligence collection on the black market should complement the existing risk management systems in Russia and elsewhere. Governments must be held responsible for securing their sensitive nuclear materials--and held accountable if they fail to do so.

Elizabeth Zolotukhina is head editor of the Case Studies Working Group with the Project on National Security Reform. Ms. Zolotukhina received her undergraduate degree from the University of Pittsburgh. Her research interests include nonproliferation and Russia.