BAGHDAD, Iraq, Oct. 24 - The Iraqi interim government has warned the United States and international nuclear inspectors that nearly 380 tons of powerful conventional explosives - used to demolish buildings, make missile warheads and detonate nuclear weapons - are missing from one of Iraq's most sensitive former military installations.

The huge facility, called Al Qaqaa, was supposed to be under American military control but is now a no man's land, still picked over by looters as recently as Sunday. United Nations weapons inspectors had monitored the explosives for many years, but White House and Pentagon officials acknowledge that the explosives vanished sometime after the American-led invasion last year.

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The bomb that brought down Pan Am Flight 103 over Lockerbie, Scotland, in 1988 used less than a pound of the same type of material, and larger amounts were apparently used in the bombing of a housing complex in November 2003 in Riyadh, Saudi Arabia, and the blasts in a Moscow apartment complex in September 1999 that killed nearly 300 people.

The explosives could also be used to trigger a nuclear weapon, which was why international nuclear inspectors had kept a watch on the material, and even sealed and locked some of it. The other components of an atom bomb - the design and the radioactive fuel - are more difficult to obtain.

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The International Atomic Energy Agency publicly warned about the danger of these explosives before the war, and after the invasion it specifically told United States officials about the need to keep the explosives secured, European diplomats said in interviews last week. Administration officials say they cannot explain why the explosives were not safeguarded, beyond the fact that the occupation force was overwhelmed by the amount of munitions they found throughout the country.

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[The] Bush administration would not allow the agency back into the country to verify the status of the stockpile. In May 2004, Iraqi officials say in interviews, they warned L. Paul Bremer III, the American head of the occupation authority, that Al Qaqaa had probably been looted. It is unclear if that warning was passed anywhere. Efforts to reach Mr. Bremer by telephone were unsuccessful.