A few days after Libya's historic pledge on Dec. 19 to abandon the quest for nuclear weapons, Libyan intelligence officials met with visiting U.S. diplomats to deliver some unsettling news: A sizable quantity of nuclear equipment purchased by Libya appeared to be missing.

The equipment -- sensitive components of machines used to enrich uranium -- had been ordered from black-market suppliers months earlier and was now long overdue, the Libyans disclosed. According to U.S. officials present at the meeting, the Libyans wanted to prepare the Americans for the possibility that more illicit nuclear shipments could suddenly appear on Tripoli's docks.

"They clearly expected more things to turn up," said one of the U.S. participants.

Four months later, the wait continues. Despite a search that has spanned the globe, U.S. and international investigators are still struggling to account for a number of sensitive parts Libya ordered for construction of its uranium enrichment plant -- parts that potentially could be used by other countries or groups seeking nuclear weapons.

The whereabouts of the parts is one of several mysteries that has preoccupied officials involved in the biggest investigation of nuclear smuggling in history -- the probe into the black-market network led by former Pakistani nuclear scientist Abdul Qadeer Khan. U.S. and U.N. investigators have identified many of the network's operatives and methods and recovered tens of thousands of parts in a dragnet that has reached from Southeast Asia to the Middle East and Europe. Yet, the investigators believe that some of the suppliers to the network have not yet been identified -- and perhaps some customers, as well.

"We haven't gotten to the bottom of the story," acknowledged one senior Bush administration official involved in the investigation. "We continue to look for, and expect to make, new discoveries. We don't think the story is fully revealed yet."

Unraveling the network and recovering missing parts and blueprints are viewed as urgent because of the possibility that nuclear technology could be diverted to unfriendly governments or terrorist groups. Yet, despite cooperation by numerous countries -- and by Khan -- the investigation has proven difficult and time-consuming.

"It is taking longer than anyone expected," said David Albright, a nuclear expert and president of the Institute for Science and International Security. "But if we don't succeed, there's a real chance the network will reconstitute itself and spread again."

Khan and a small group of business associates were the architects of the trading network, which coordinated the manufacture and shipment of nuclear components from as many as a dozen locations to Libya, North Korea, Iran and possibly other countries. Although the smuggling ring traded mostly in components for gas centrifuges -- complex machines used to enrich uranium for nuclear weapons -- the network is also known to have supplied uranium and nuclear weapons blueprints to Libya.

The dramatic decision by Libyan leader Moammar Gaddafi in December to renounce his pursuit of nuclear weapons brought the Khan network to light and provided investigators with clues that led to the discovery of suppliers and shipping routes, according to U.S. officials and documents.

In recent weeks, investigators for the U.N. nuclear watchdog, the International Atomic Energy Agency, have zeroed in on newly discovered suppliers as well as a handful of manufacturing centers described by investigators as workshops for centrifuge parts. Of recently heightened interest to investigators are a group of Turkish businesses that appear to have both manufactured electronic parts for centrifuges and assembled other components for shipment to Libya, according to weapons experts and diplomats familiar with the investigation.

IAEA officials declined to comment on specific findings. "There is an intense investigation underway by the IAEA into the entire Khan network," said spokeswoman Melissa Fleming. "We are systematically conducting interviews with all the members and suppliers."

At the same time, investigators are continuing to interview known suppliers and pass questions to Khan through Pakistani intermediaries in an effort to establish precisely the types and quantities of parts produced by the network and what happened to them.

Based on those interviews, IAEA and U.S. officials now believe that Libya's uranium enrichment program was not nearly as advanced as first believed. U.N. inspectors in Vienna said on Friday that Libya's enriched uranium came from a Pakistani black market supply hub that also supplied Iran. Iran has insisted that uranium traces on its centrifuges were not proof of a weapons program.

Libyan officials have acknowledged buying parts for at least 4,000 advanced centrifuges known as P2s -- machines that, when assembled, would have given Libya the capability to produce enough enriched uranium for several nuclear bombs a year. Now, it appears that Libya received only a fraction of the parts needed to build the machines.

Some of the most sensitive parts never arrived there and were not part of the shipment of nuclear parts intercepted by U.S., British and Italian authorities last October, according to U.S. and European officials close to the investigation. "The Libyans did not have the parts to assemble even a single [P2] centrifuge," said one diplomat familiar with the IAEA's probe. "They received thousands of parts, but lacked many of the ones considered to be the most sensitive."

Among the missing components were rotors, the rapidly spinning tubes that make up the core of the centrifuge, as well as bellows, rotor caps and other parts made from a high-strength metal known as maraging steel, U.S. and European sources confirmed. Libyan officials have told investigators they had expected to receive all the parts and in some cases had visited facilities where the components were said to be made.

In the case of the missing rotors, investigators see three possibilities: They were being manufactured by unknown suppliers who have not yet come to light, they were never made or they were shipped and then diverted to another country. Libya did acquire machine tools and high-strength metals that could be used to make rotors, although such an undertaking would have required years to complete and substantial outside help.

One additional shipment of centrifuge parts did turn up in Libya in March, unnoticed by anyone until Libyan authorities discovered the parts and surrendered them to U.S. officials.

In other instances, investigators have obtained documents or intelligence pointing to the existence of parts that were intended for Libya and cannot be traced. Officials familiar with the evidence declined to elaborate, saying they feared the investigation might be compromised.

"The numbers are probably small," said a U.S. intelligence official involved in the probe. "But it's hard to assess the seriousness because we don't know what we don't know."

Adding to the mystery, investigators have discovered discrepancies between the number of centrifuge parts requested by Libya and the quantities the Khan network made. In some cases the production exceeded the demand by a considerable margin, fueling concerns that the smuggling ring had other customers that have not come to light.

"The numbers don't match up," said the diplomat familiar with the IAEA probe, "and we're not yet sure what it means."

Abdul Qadeer Khan, the founder of Pakistan's nuclear program, led a smuggling ring whose activities have proven difficult to unravel.