ZURICH, Switzerland — Critical components and specialized tools destined for Libya's nuclear weapons program disappeared before arrival in 2003 and international investigators now suspect that they were diverted to another country, according to court records and investigators.
Efforts to find the missing equipment have led to dead ends, raising what investigators said was the strong likelihood that the sophisticated material was sold to an unidentified customer by members of the international smuggling ring that had been supplying nuclear technology and weapons designs to Libya.
The equipment -- components for advanced centrifuges, along with material and precision tools to manufacture more of them -- does not constitute an immediate threat, but nuclear experts said it would cut years off an effort to enrich uranium for an atomic bomb.
The mystery of the missing high-tech equipment illustrates both the extensive knowledge investigators have gained about the smuggling operation and the troubling gaps that remain. It also raises the question of whether a rogue nation or group might be secretly building a nuclear weapon.
Two senior international investigators said that the illicit technology had been shipped to Turkey, Malaysia and Dubai, United Arab Emirates, before it disappeared and that it remained unaccounted for.
The equipment was initially meant for a $100-million, clandestine uranium enrichment plant and bomb factory being built for Libya by Pakistani scientist Abdul Qadeer Khan and a network of middlemen on three continents.
The seizure by the United States and Britain of a separate shipment of nuclear-related components from a freighter headed for Libya in October 2003 crippled the network and led to Khan's admission that he had been selling know-how and technology to Iran, North Korea and Libya.
Since then, the biggest concerns for international inspectors and intelligence agencies examining Khan's operation have been whether an unidentified customer is also pursuing a nuclear weapon or whether Iran might have received the missing technology and, potentially, designs for an atomic weapon.
Investigators said business records and interviews with some participants in the ring suggested the existence of a customer other than Libya. They said the vanished equipment, though not proof, constituted the strongest clue yet.
The list of potential clients for a nuclear weapons program is not long, but it extends to countries in the Middle East, Africa and Southeast Asia. Khan traveled extensively and had contacts worldwide. Investigators have identified at least 30 companies and middlemen who sold goods to his network.
"For sure there were other customers," said one of the senior investigators, who spoke on condition that his name be withheld because the inquiry is ongoing. "We just don't know who and we don't know how far along they might be."
U.S. and Israeli government officials have publicly accused Tehran of operating a nuclear weapons program, and intelligence officials from both countries have suggested that Iran might have hidden installations.
A non-Western intelligence official said it was possible that the missing centrifuge components and other material was sold secretly to Iran by someone in the Khan network as the operation started to unravel after the seizure of the shipment in 2003.
Iran insists that its nuclear program is intended only to generate electricity and that it has opened all of its atomic-related installations to inspectors from the International Atomic Energy Agency, the United Nations' nuclear watchdog.
The IAEA has been inspecting Iranian installations for nearly two years and says it has uncovered no proof that Iran is working on weapons.
But concerns remain. Last month, the agency's deputy director complained publicly that Tehran had not turned over all information related to a 1987 meeting in which Khan's associates offered to sell weapons designs to Iran.
The complexity of enriching uranium makes it likely that an unknown customer would be a country rather than a terrorist group. Intelligence officials said it was unlikely that even a sophisticated terrorist group would have the resources for such an undertaking, though they could not rule it out.
The primary investigation of the Khan network is being conducted by a small team of nuclear experts from the IAEA's headquarters in Vienna. Intelligence officials and prosecutors from other countries, including the United States, Germany, Britain, France, South Africa and Switzerland, are involved.
Seven people alleged to have ties to the ring have been taken into custody in Dubai, Germany, Malaysia, South Africa and Switzerland, but none has gone to trial.
Khan, 68, regarded as a national hero for helping build his country's atomic weapons, was pardoned by Pakistan for selling its nuclear secrets. He remains under house arrest, outside the reach of foreign investigators.