(AP) Looters unleashed last year by the U.S.-led invasion of Iraq overran a sprawling desert complex where a bunker sealed by U.N. monitors held old chemical weapons, American arms inspectors report.

Charles Duelfer's arms teams say all U.N.-sealed structures at the Muthanna site were broken into. If the so-called Bunker 2 was breached and looted, it would be a new case of restricted weapons being at risk of having fallen into militants' hands.

Separately, Human Rights Watch said Saturday it alerted the U.S. military to a cache of hundreds of warheads containing high explosives in Iraq in May 2003, but that officials seemed disinterested and still hadn't secured the site 10 days later, even though it was being looted every day by armed men.

The disclosure, made by a senior leader of the New York-based group, raised new questions about the willingness or ability of U.S.-led forces to secure known stashes of dangerous weapons in Iraq.

Peter Bouckaert, who heads Human Rights Watch's international emergency team, told The Associated Press he was shown two rooms "stacked to the roof" with surface-to-surface warheads on May 9, 2003, in a warehouse on the grounds of the 2nd Military College in Baqouba, 35 miles northeast of Baghdad.

Bouckaert said he gave U.S. officials the exact location of the warheads, but that by the time he left the area on May 19, 2003, he had seen no U.S. forces at the site, which he said was being looted daily by armed men.

His comments came as the question of 377 tons of high explosives reported missing from another site - the Al-Qaqaa military installation south of Baghdad - has become a heated issue in the final days of the U.S. presidential campaign.

Officials are unsure whether the episode at Muthanna points to a threat of chemical attack, since it isn't known if usable chemical warheads were in the bunker, what may have been taken, or by whom.

"Clearly, there's a potential concern, but we're unable to estimate the relative level of it because we don't know the condition of the things inside the bunker," said Ewen Buchanan, spokesman for the U.N. arms inspection agency in New York, whose specialists have been barred from Iraq since the invasion.

Chief arms hunter Duelfer told The Associated Press by e-mail Friday from Iraq that he was unaware of "anything of importance" looted from the chemical weapons complex. The report his Iraq Survey Group issued on Oct. 6 said, however, that it couldn't vouch for the fate of old munitions at Muthanna, a 35-square-mile complex in the heart of the embattled "Sunni Triangle."

One chemical weapons expert said even old, weakened nerve agents - in this case sarin - could be a threat to unprotected civilians.

The weapons involved would be pre-1991 artillery rockets filled with sarin, or their damaged remnants - weapons that were openly declared by Iraq and were under U.N. control until security fell apart with the U.S. attack. They are not concealed arms of the kind President Bush claimed Iraq had, but which were never found.

In its Oct. 6 report, summarizing a fruitless search for banned weapons in Iraq, Duelfer's group disclosed that widespread looting occurred at Muthanna, 35 miles northwest of Baghdad, in the aftermath of the fall of the Iraqi capital in April 2003.

A little-noted annex of the 985-page report said every U.N.-sealed location at the desert installation had been breached in the looting spree, and "materials and equipment were removed."

Bunker 2 at Muthanna State Establishment, once Iraq's central chemical weapons production site, was put under U.N. inspectors' control in early 1991 after it was heavily damaged by a U.S. precision bomb in the first Gulf War. At the time, Iraq said 2500 sarin-filled artillery rockets had been stored there.

The U.N. teams sealed up the bunker with brick and reinforced concrete, rather than immediately attempt the risky job of clearing weapons or remnants from under a collapsed roof and neutralizing them.

A CIA analysis, not done on site, hypothesized in 1999 that all the sarin must have been destroyed by fire. But a U.S. General Accounting Office review last June questioned that analysis, and the United Nations, whose teams were there, said the extent of destruction was never determined.

Buchanan said a U.N. team inspected the sealed Muthanna bunker on Dec. 4, 2002, and inspectors continued to visit Muthanna up to March 14, 2003, although they did not view the bunker that day. Four days later, on the eve of the U.S. invasion, the U.N. monitors had to leave Iraq.

As for when the sealed bunker may have been breached, the report said, "The facilities at the southern section" - the bunker area - "were removed by unknown entities between April and June 2003." It didn't elaborate, but presumably the first U.S. search teams arrived at Muthanna in June and discovered the looting.

"The (Iraq Survey Group) is unable to unambiguously determine the complete fate of old munitions, materials and chemicals produced and stored there," the Duelfer report said.

The three-week-old report also said, without elaboration, that chemical munitions "are still stored there" and that warheads, apparently not filled with chemical agent, "are still being looted."

As for the Baqouba facility, Human Rights Watch's Bouckaert said displaced people he was working with in the area had taken him to the warheads. "They said, `There's stocks of weapons here and we're very concerned - can you please inform the coalition?"' he said in a telephone interview from South Africa.

After photographing the warheads, Bouckaert said he went straight to U.S. officials in Baghdad's Green Zone complex, where he claimed officials at first didn't seem interested in his information.

"They asked mainly about chemical or biological weapons, which we hadn't seen," he said. "I had a pretty hard time getting anyone interested in it."

Bouckaert said he eventually was put in touch with unidentified U.S. officials and showed them on a map where the stash was located, also giving them the exact GPS coordinates for the site.

But he said he never saw U.S. forces at the site when he returned to the area for daily interviews with refugees, and that the site still was not secured when he finally left the area.

"For the next 10 days I continued working near this site and going back regularly to interview displaced people, and nothing was done to secure the site," he said.

"Looting was taking place by a lot of armed men with Kalashnikovs and rocket-propelled grenades," Bouckaert said. He said each of the warheads contained an estimated 57 pounds of high explosives.

"Everyone's focused on Al-Qaqaa, when what was at the military college could keep a guerrilla group in business for a long time creating the kinds of bombs that are being used in suicide attacks every day," he said.

The International Atomic Energy Agency said Monday that Iraq had reported 377 tons of high explosives missing from al-Qaqaa "due to a lack of security" at the vast site 30 miles south of Baghdad.

Iraqi officials told the agency the explosives - which can be used to make the kind of car bombs that insurgents have used in numerous attacks on U.S.-led forces - went missing amid looting after the April 9, 2003 fall of the Iraqi capital.

The Pentagon has suggested the explosives, which can be used to make the kind of car bombs that insurgents have used in numerous attacks on U.S.-led forces, may have been removed before U.S. forces moved into the area.

U.S. Army Maj. Austin Pearson said Friday that his team removed 250 tons of plastic explosives and other munitions from al-Qaqaa on April 13, 2003. But those munitions were not located under U.N. nuclear agency seal as the missing high-grade explosives had been, and the Pentagon was unable to say definitively that they were part of the missing 377 tons.

Bouckaert, who last year criticized U.S. officials for not acting on important information about mass graves in Iraq, said he estimates there were between 500 and 1,000 tons of high explosive warheads at the war college site in Baqouba.

The site also included anti-tank mines and anti-personnel mines, he said.

Car bombs require only about 6 1/2 pounds of explosives, meaning each warhead potentially could have yielded enough material for nine bombs, Human Rights Watch said.

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