The break for American intelligence operatives tracking Abdul Qadeer Khan's nuclear network came in the wet August heat in Malaysia, as five giant cargo containers full of specialized centrifuge parts were loaded into one of the nondescript vessels that ply the Straits of Malacca.
The C.I.A. had penetrated the factory of Scomi Precision Engineering, where one of the nuclear network's operatives -- known to the workers only as Tinner -- watched over the production of the delicate machinery needed to enrich uranium for nuclear bombs.
Spy satellites tracked the shipment as it wended its way to Dubai, where it was relabeled ''used machinery'' and transferred to a German-owned ship, the BBC China. When it headed through the Suez Canal, bound for Libya, the order went out from Washington to have it seized, according to accounts from American officials.
That seizure led to the unraveling of a trading network that sent bomb-making designs and equipment to at least three countries -- Iran, North Korea and Libya -- and has laid bare the limits of international controls on nuclear proliferation.
Yesterday, President Bush proposed to enhance that system by restricting the production of nuclear fuel to a few nations. [Page A18.]
The scope and audacity of the illicit network are still not fully known. Nor is it known whether the Pakistani military or government, which had supported Dr. Khan's research, were complicit in his activities.
But what has become clear in recent days is that Dr. Khan, a Pakistani national hero who began his rise 30 years ago by importing nuclear equipment to secretly build his country's atom bomb, gradually transformed himself into the largest and most sophisticated exporter in the nuclear black market.
''It was an astounding transformation when you think about it, something we've never seen before,'' said a senior American official who has reviewed the intelligence. ''First, he exploits a fragmented market and develops a quite advanced nuclear arsenal. Then he throws the switch, reverses the flow and figures out how to sell the whole kit, right down to the bomb designs, to some of the world's worst governments.''
The story of that transformation emerges from recent interviews on three continents -- from Islamabad, Pakistan's capital, to Kuala Lumpur, Malaysia; from the streets of Dubai, where many of the deals were cut, to Washington and Vienna, where intelligence agencies and the International Atomic Energy Agency struggled to understand and defuse the threat.
Taken together, they show how Dr. Khan assembled a far-reaching organization of scientists, engineers and business executives who operated on murky boundaries between the legal and the illegal, sometimes underground but often in plain view, unencumbered by international agreements that prohibit trafficking in nuclear technology.
Dr. Khan started in the mid-1980's, according to nuclear proliferation experts, by ordering twice the number of parts the Pakistani nuclear program needed, and then selling the excess to other countries, notably Iran.
Later, his network acquired another customer: North Korea, which was desperate for a more surreptitious way to build nuclear weapons after the United States had frozen the North's huge plutonium-production facilities in Yongbyon.
And in the end he moved on to Libya, his ultimate undoing, selling entire kits, from centrifuges to enrich uranium, to crude weapons designs. Investigators found the weapons blueprints wrapped in bags from an Islamabad dry cleaner.
In his speech yesterday, Mr. Bush said the network even sold raw uranium to be processed into bomb fuel. He also identified Dr. Khan's deputy -- ''the network's chief financial officer and money-launderer,'' he called him -- as Bukhari Sayed Abu Tahir, a businessman in Dubai, who, investigators say, placed the order for the Libyan equipment.
One longtime trading partner of Dr. Khan's was Peter Griffin, a British engineer who said in an interview that he had been a supplier to Pakistan for two decades, in the period when Dr. Khan was building nuclear weapons.