Did rogue network leak nuclear bomb design?

The Pakistanis, too, were upset, according to the Albright study, because they recognized that the design was most likely their own. Although Pakistani warheads are also derived from Chinese originals, Pakistan has gone much further in developing the sophisticated electronics and triggering mechanisms necessary for smaller designs.

The Pakistanis "were genuinely shocked; Khan may have transferred his own country's most secret and dangerous information to foreign smugglers so that they could sell it for a profit," writes Albright.

In some ways, possessing a sophisticated warhead design would not help would-be nuclear proliferators all that much.

For one thing, it might take only a crude bomb, even thought it might be as bulky as a truck, to provide the strategic deterrent power of nuclear weapons. For another, designing a bomb is not the hardest part of obtaining a bomb. Producing or obtaining the fissile material necessary for the weapon's explosive heart remains a more daunting challenge.

Still, nuclear engineering is difficult. North Korea's nuclear test of 2006 was widely considered by Western scientists to be a fizzle. Even US national labs have occasionally designed warhead duds.

"It's tough to really get all those parts working right," says Charles Ferguson, a nuclear expert and fellow for science and technology at the Council on Foreign Relations in Washington.

And Iran and North Korea might have special reason to desire a small warhead, one that would fit atop medium-range missiles they are already developing, threatening neighbors and turning them instantly into regional superpowers.

Clearly, the US does not yet quite understand the reach of the modern model of proliferation: nonstate actors, motivated by profit, drawing on sources and contacts in many nations.

"Even if the Khan network itself is shut down, there still are nuclear black markets going on," says Mr. Ferguson.

Ferguson and other experts say that there is no public evidence that the US yet has pressured Pakistan to allow US investigators to personally interview Khan.

Khan remains under house arrest in Pakistan. On June 16 he denied being the source of the design found in the Tinners' possession.

"This is all a lie, there is no truth to it," Khan told Agence France-Presse by phone from Islamabad.

While the US can be confident that it has gotten at least a partial picture of the Khan network through indirect sources, sitting down in a room with the mastermind for a concerted interrogation could still be invaluable, says Mr. Wolfsthal of CSIS.

The Bush administration has not pushed for such access because Khan remains a hero to many Pakistanis, and the White House does not want to do anything to further risk the position of President Pervez Musharraf, says Wolfsthal.

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