There appear to be two periods of time when the 377 tons of high explosives missing from a military facility in Iraq could have been moved or stolen - in the weeks before the U.S. invasion began or several weeks in April after U.S. troops overran the Al-Qaqaa base and moved on to Baghdad.

Iraqi officials told the International Atomic Energy Agency two weeks ago that the explosives vanished as a result of "theft and looting ... due to lack of security."

The ministry's letter said the explosives were stolen sometime after coalition forces took control of Baghdad on April 9, 2003. But they have not been able to explain how they know when the explosives were removed from bunkers sealed by the IAEA as part of the weapons inspection program.

The disappearance of the explosives has raised questions about why the United States didn't do more to secure the facility and failed to allow full international inspections to resume after the invasion. It has also become an issue in the U.S. presidential campaign.

The Kerry campaign called the disappearance the latest in a "tragic series of blunders" by the Bush administration in Iraq. The White House has issued a statement saying that the matter is under investigation and the explosives may have been moved before the invasion.

IAEA Inspectors report that they checked the seals placed on the bunker storing stockpiles of HMX and RDX kept at al-Qaqaa on March 9. There have been reports that another IAEA team checked the site on March 15, but that has yet to be confirmed.

Until March 20, when coalition forces attacked Iraq, the explosives could have been moved to another location without risk of being intercepted by U.S. air or ground forces. U.S. attack jets patrolled Iraq's major highways after March 19 in search of military targets to destroy.

Iraqi forces were still at the al-Qaqaa complex on April 3 when Task Force 3-15 of the 2nd Brigade, 3rd Infantry Division arrived, said Col. Dave Perkins, who commanded the brigade.

Troops reported seeing the gates open and several hundred troops were defending the site, Perkins said Wednesday.

The infantrymen's mission was to destroy any Iraqi troops in the area and, after defeating the forces in the facility, the task force moved north on April 6 to prepare for the assault on Baghdad. The 2nd Brigade, 3rd Infantry Division was the first U.S. unit to enter central Baghdad on April 7.

U.S. troops passed through more than a dozen Iraqi military facilities on the march from the Kuwait border to Baghdad, and while cursory checks were made for suspected weapons of mass destruction, large ammunition, weapons and explosives dumps were left unguarded. Commanders reported the coordinates of these sites to higher headquarters so rearguard troops could take care of them.

Perkins, now a staff officer at the Pentagon, said that while some looting at the site had taken place, a large-scale operation to remove the explosives using multi-ton trucks would almost certainly have been detected.

There were virtually no civilian vehicles on the roads in late March and early April. U.S. military police patrolled the major roads, which U.S. forces used extensively to resupply the troops in Baghdad.

Perkins described Iraq as littered with weapons, and the Qaqaa base was one munitions depot among many. Many other depots his forces found had been cleaned out, with weapons scattered, presumably so they wouldn't be destroyed by airstrikes.

"We came upon a lot of these sites," he said. "What we found all the way up was dispersed munitions, dispersed weapons."

The ammunition storage areas were not inspected until May 27, when the team reported widespread looting of the entire complex. This team reported that the IAEA seals and the high explosives were missing.

Throughout Iraq, civilians looted military facilities for anything of potential value between April 11 and May 27, when there were not enough U.S. troops on the ground to stop them.

High explosives, such as HMX and RDX, do not explode when exposed to flame, rather burn at a high temperature, which makes them useful for cooking.