The Pentagon today suggested that up to two-thirds of the missing Iraqi high explosives that have stirred controversy in the current election campaign were actually destroyed by a U.S. Army unit last year, but a unit commander presented at a press conference to buttress that claim said he could not confirm that the ordnance he seized included the explosives in question.

Army Maj. Austin Pearson, who commanded an ordnance company in the 3rd Infantry Division during last year's invasion of Iraq, said his unit removed 250 tons of TNT, plastic explosives, detonation cords and white-phosphorous rounds from the Qaqaa storage site 30 miles south of Baghdad on April 13, 2003. He said the munitions were later destroyed.

The International Atomic Energy Agency reported Monday that 377 tons of high explosives -- including compounds known as HMX and RDX -- were missing from the Qaqaa site. HMX can be used to trigger nuclear weapons.

Pentagon spokesman Lawrence Di Rita told the press conference that "plastic explosives" was a term commonly used for RDX, one of three types of high-powered explosives that were placed under seal at the Qaqaa site by the IAEA.

Di Rita said, "We believe that some of the things they [U.S. troops] were pulling out of there were RDX." But he acknowledged that this could not be confirmed, and he said further study was needed.

The suggestion that RDX was among the explosives destroyed by Pearson's unit also appeared to be at odds with an ABC News video, shot by an embedded reporter, that showed soldiers of the Army's 101st Airborne Division cutting a wire seal on a bunker and inspecting explosives stored inside on April 18, five days after Pearson said his unit removed munitions. Weapons experts said the wire seal shown on the video was placed there by the IAEA and that the explosives inside the bunker were of the kinds that have been reported missing.

"I did not see any IAEA seals at any of the locations we went into," Pearson said at the press conference. "I was not looking for that."

Di Rita sought to play down the significance of the missing explosives, noting that they amounted to less than "one-thousandth" of the 400,000 tons of Iraqi munitions of all types that the Pentagon says have been seized by U.S. forces since the invasion.

The missing explosives are believed to have been removed last year sometime between March 15 -- the last time U.N. inspectors visited the Qaqaa site -- and May 8, when a U.S. weapons-hunting unit began a thorough search of the vast compound and failed to find any explosives that had been quarantined by the IAEA.

The U.S.-backed Iraqi interim government has said the explosives disappeared after U.S. forces toppled the government of Saddam Hussein on April 9, 2003. The government has attributed the loss to looting amid lax security. The Bush administration has suggested that the munitions were removed by the Hussein regime before the Americans took over.

Democratic presidential candidate John F. Kerry has seized on the issue to buttress his charges that Bush mismanaged the war, failing to ensure that depots of dangerous weapons and ammunition were secured and not dispatching sufficient troops and equipment to accomplish that task.

In a statement issued by the Kerry campaign today, retired Army Gen. Wesley K. Clark said the Pentagon briefing raised more questions than it answered and only added to confusion over "conflicting explanations for why hundreds of tons of explosives, munitions and weapons were not properly secured in Iraq."

He said, "President Bush needs to address this issue and level with the American people and our troops about why protecting these dangerous materials was not a high priority. It is shameful that George Bush remains silent, letting our troops take the blame for his failed leadership."

In television interviews last night and this morning, David Kay, the former chief U.S. weapons inspector in Iraq, confirmed that, based on a videotape aired by ABC News last night, the high explosives were still inside a sealed bunker when U.S. troops entered it on April 18, 2003. The video shows soldiers cutting a thin wire seal that Kay said was used by the IAEA, entering the bunker and looking at explosives in one of scores of barrels and crates.

"With this one, I think it is game, set, and match," Kay said on CNN last night. "There was HMX, RDX in there. The seal was broken. And quite frankly, to me the most frightening thing is not only was the seal broken, lock broken, but the soldiers left after opening it up. I mean, to rephrase the so-called pottery barn rule. If you open an arms bunker, you own it. You have to provide security."

Kay said the soldiers likely did not know the explosives were HMX, "which speaks to lack of intelligence given troops moving through that area, but they certainly knew they had explosives." He added that "Iraq is awash with tens of thousands of tons of explosives right now in the hands of insurgents because we did not provide the security when we took over the country."

This morning, in another interview on CNN, Kay disputed Defense Secretary Donald H. Rumsfeld's assertion that looters probably could not have moved such a relatively large amount of high explosives out of the Qaqaa storage site after U.S. forces went through.

"Well, with all due respect to the secretary, looters don't come in convoys," Kay said. "They're more like ants in your kitchen. They're every place, and they move relatively individually, relatively small amounts. The total is huge. I've been on at least 15 sites while looting was going on in Iraq, and they take down buildings, they move heavy equipment. It's amazing what they can move. Now looting, obviously, occurred at that site. We observed it after the fact. I'm afraid that is the explanation."

The amount of missing explosives is "insignificant" compared to the huge amounts of munitions scattered around Iraq, "but it is indicative of what occurred across the country," Kay said. "U.S. military personnel in Iraq every day are being killed by weapons that they [insurgents] didn't bring into Iraq from abroad. They stole from Iraqi arms dumps that were not guarded." He said the ABC video, shot by a television crew from an ABC affiliate in Minnesota, shows a "disturbing" example of "military tourism within a war zone," and that "if you open up a bunker, you're responsible for guarding it."

The most powerful of the explosives stored at the Qaqaa site -- HMX -- can be used in a trigger for nuclear devices, which is why the material was under IAEA seal. Defense officials say car bombs and roadside explosive devices used against U.S. forces and other foreigners in Iraq have tended to be made from old artillery shells and other common munitions.

As Democrats continued to raise the issue of the missing explosives, reporters today peppered White House press secretary Scott McClellan with questions about what happened to them, Washington Post staff writer Mike Allen reported. McClellan held out the prospect that U.S. troops might have destroyed the explosives, as the Pentagon had tried to suggest.

"It's a real possibility that . . . these explosives were part of the munitions that were destroyed," he said. "The facts are not known."

At an afternoon rally in Diamondale, Mich., Cheney also referred to the news from the Pentagon briefing that 250 tons of ammunition had been removed by U.S. troops from the Qaqaa complex, asserting that it proved Kerry has been twisting the truth, Washington Post staff writer Lyndsey Layton reported.

"Our troops were doing their jobs then and they continue to do their jobs now," Cheney said, triggering shouts of "USA! USA!" from the men, women and Boy Scouts gathered in a sports complex in central Michigan.