Investigators have discovered that the nuclear weapons designs obtained by Libya through a Pakistani smuggling network originated in China, exposing yet another link in a chain of proliferation that stretched across the Middle East and Asia, according to government officials and arms experts.

The bomb designs and other papers turned over by Libya have yielded dramatic evidence of China's long-suspected role in transferring nuclear know-how to Pakistan in the early 1980s, they said. The Chinese designs were later resold to Libya by a Pakistani-led trading network that is now the focus of an expanding international probe, added the officials and experts, who are based in the United States and Europe.

The packet of documents, some of which included text in Chinese, contained detailed, step-by-step instructions for assembling an implosion-type nuclear bomb that could fit atop a large ballistic missile. They also included technical instructions for manufacturing components for the device, the officials and experts said.

"It was just what you'd have on the factory floor. It tells you what torque to use on the bolts and what glue to use on the parts," one weapons expert who had reviewed the blueprints said in an interview. He described the designs as "very, very old" but "very well engineered."

U.S. intelligence officials concluded years ago that China provided early assistance to Pakistan in building its first nuclear weapon -- assistance that appeared to have ended in the 1980s. Still, weapons experts familiar with the blueprints expressed surprise at what they described as a wholesale transfer of sensitive nuclear technology to another country. Notes included in the package of documents suggest that China continued to mentor Pakistani scientists on the finer points of bomb-building over a period of several years, the officials said.

China's actions "were irresponsible and short-sighted, and raise questions about what else China provided to Pakistan's nuclear program," said David Albright, a nuclear physicist and former U.N. weapons inspector in Iraq. "These documents also raise questions about whether Iran, North Korea and perhaps others received these documents from Pakistanis or their agents."

The package of documents was turned over to U.S. officials in November following Libyan leader Moammar Gaddafi's decision to renounce weapons of mass destruction and open his country's weapons laboratories to international inspection. The blueprints, which were flown to Washington last month, have been analyzed by experts from the United States, Britain and the Vienna-based International Atomic Energy Agency, the U.N. nuclear watchdog.

Weapons experts in Libya also found large amounts of equipment used in making enriched uranium, the essential ingredient in nuclear weapons. That discovery helped expose a rogue nuclear trading network that officials say funneled technology and parts to Libya as well as Iran and North Korea. A central figure in the network, Pakistani metallurgist Abdul Qadeer Khan, acknowledged in a televised confession last month that he had passed nuclear secrets to others. Pakistan's president, Gen. Pervez Musharraf, then pardoned Khan.

Of the many proliferation activities linked to Khan's network, the selling of weapon designs is viewed as the most serious. The documents found in Libya contained most of the information needed to assemble a bomb, assuming the builder could acquire the plutonium or highly enriched uranium needed for a nuclear explosion, according to U.S. and European weapons experts familiar with the blueprints. At the same time, one of the chief difficulties for countries trying to build nuclear weapons has been obtaining the plutonium or uranium.

Libya appeared to have made minimal progress toward building a weapon, and had no missile in its arsenal capable of carrying the 1,000-pound nuclear device depicted in the drawings, the officials said. However, weapons experts noted, the blueprints would have been far more valuable to the other known customers of Khan's network.

"This design would be highly useful to countries such as Iran and North Korea," said Albright, whose Washington-based Institute for Science and International Security has studied the nonconventional weapons programs of both states. The design "appears deliverable by North Korea's Nodong missile, Iran's Shahab-3 missile and ballistic missiles Iraq was pursuing just prior to the 1991 Persian Gulf War," he said.

Such a relatively simple design also might be coveted by terrorist groups who seek nuclear weapons but lack the technical sophistication or infrastructure to build a modern weapon, said one Europe-based weapons expert familiar with the blueprints. While such a bomb would be difficult to deliver by air, "you could drive it away in a pickup truck," the expert said.

The device depicted in the blueprints appears similar to a weapon known to have been tested by China in the 1960s, officials familiar with the documents said. Although of an older design, the bomb is an implosion device that is smaller and more sophisticated than the bombs dropped on Hiroshima and Nagasaki at the end of World War II. Implosion bombs use precision-timed conventional explosives to squeeze a sphere of fissile material and trigger a nuclear chain reaction.

Pakistan's first nuclear test in 1998 involved a more modern design than the one sold to Libya. Albright said the Libyan documents "do not appear to contain any information about the nuclear weapons Pakistan has built."

The documents at the center of the investigation were handed over to IAEA inspectors in two white plastic shopping bags from a Pakistani clothing shop. The shop's name -- Good Looks Tailor -- and Islamabad address were printed on the bags in red letters. One of the bags contained drawings and blueprints of different sizes; the other contained a stack of instructions on how to build not only a bomb but also its essential components.

The documents themselves seemed a hodgepodge -- some in good condition, others smudged and dirty; some professionally printed, others handwritten. Many of the papers were "copies of copies of copies," said one person familiar with them. The primary documents were entirely in English, while a few ancillary papers contained Chinese text. The package also included open-literature articles on nuclear weapons from U.S. weapons laboratories, officials familiar with the documents said.

Strikingly, although most of the essential design elements were included, a few key parts were missing, the officials and experts said. Some investigators have speculated that the missing papers could have been lost, or hadn't yet been provided -- possibly they were being withheld pending additional payments. Others suggested that the drawings were simply thrown in as a bonus with the purchase of uranium-enrichment equipment -- "the cherry on the sundae," one knowledgeable official said.

Libyan scientists interviewed by international inspectors about the designs said they had not seriously studied them and were unaware that anything was missing. As Libya had no suitable missile or delivery system for a nuclear weapon, the scientists might have decided to delay work on bomb designs until other parts of their weapons program were further advanced, one knowledgeable U.S. official said.

U.S. and European investigators said there were many similarities among the other nuclear-related designs and components found in Libya and Iran, suggesting they were provided by the same network.

As for who delivered the material to the Libyans, a European official who has studied the question said the connection to the Khan network was indirect. "The middleman is quite invisible. The middleman has covered his tracks very well."

The evidence of China's transfer of nuclear plans to Pakistan confirms something that U.S. officials have believed since at least the early 1980s. A declassified State Department report on Pakistan's nuclear program written in 1983 concluded that China had "provided assistance" to Pakistan's bomb-making program. "We now believe cooperation has taken place in the area of fissile material production and possibly nuclear device design," the report said.

While the discovery of direct evidence of such cooperation was disturbing, it was noteworthy that China's views on proliferation have changed dramatically since the 1980s, and its leaders now generally cooperate with the United States and other countries in stopping the leaking of sensitive weapons technology, said Jonathan Wolfsthal, a nonproliferation expert at the Carnegie Endowment for International Peace.

"Did the Chinese make a huge mistake in sharing technology with Pakistan? Sure. Did we make a mistake by looking the other way in the 1980s when Pakistan was developing the bomb? Yes," Wolfsthal said. "But none of that should get in the way of dealing with the real threats we face today. Our priority must be to drain the swamp created by the action of these nuclear suppliers and businessmen over the past 10 years."

Researcher Lucy Shackelford contributed to this report.

Pakistani metallurgist Abdul Qadeer Khan confessed on television that he had passed on nuclear secrets.