Washington ? 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 seemed 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.”