WASHINGTON - Four years after Abdul Qadeer Khan, the leader of the world's largest atomic black market, was put under house arrest and his operation declared over, international inspectors and Western officials were confronting a new mystery left by him, this time over who might have received blueprints for a sophisticated and compact nuclear weapon found on his network's computers.
Working in secret for two years, investigators have tracked the digitized blueprints to Khan computers in Switzerland, Dubai, Malaysia and Thailand. The blueprints are electronic and could be rapidly reproducible.
The revelation this weekend that the operation even had such a blueprint underscores the questions that remain about what Khan, a Pakistani metallurgist and the father of Pakistan's nuclear weapons program, was selling and to whom. It also raises the possibility that he might still have sensitive material in his possession. Yet even as inspectors and intelligence officials press their investigation of Khan, officials in Pakistan have declared the scandal over and have openly discussed the possibility of setting him free. In recent weeks, American officials have privately warned the new government in Pakistan about the dangers of doing so.
"We've been very direct with them that releasing Khan could cause a world of trouble," one senior administration official who has been involved in the effort said last week. "The problem with Pakistan these days is that you never know who is making the decision - the army, the intelligence agencies, the president or the new government."
The illicit nuclear network run by Khan was broken up in early 2004. President Bush declared that shattering the operation was a major intelligence coup for the United States. Since then, evidence has emerged that the network sold uranium enrichment technology to Iran, North Korea and Libya, and investigators are still pursuing leads that he may have done business with other countries as well.
Khan is an expert in centrifuges used to produce enriched uranium for bomb fuel, and much of the technology he sold involved enrichment. But it was only in recent days that officials disclosed that they had found the electronic design for a bomb among material seized from some of Khan's top lieutenants, members of the Tinner family of Switzerland.
The same design documents were found on computers in three other locations connected to Khan operatives.
American officials and inspectors at the International Atomic Energy Agency say they have been unable to determine whether the weapon blueprints were sold to Iran or the smuggling ring's other customers.
Pakistani officials have balked at providing much information about the newly revealed warhead design, just as they have refused to allow the CIA or international atomic inspectors to directly interrogate Khan, who is still considered a national hero in Pakistan for helping it acquire nuclear weapons.
Pakistani officials insist that Khan, as leader of a uranium enrichment program, had no weapons access. But this is the second weapons design found in his smuggling network. The first was for an unwieldy but effective Chinese design from the mid-1960s that Libya acknowledged obtaining from the Khan network before it surrendered its bomb-making equipment in 2003.
Both the new and old designs exploit the principle of implosion, in which a blast wave from a sphere of conventional explosives squeezes inward with tremendous force to compress a ball of bomb fuel, starting the chain reaction and the atomic explosion.
A nuclear official in Europe familiar with the Khan investigation said the new design was powerful but miniaturized - using about half the uranium fuel of the older design to produce a greater explosive force.