An arms dump blast that killed eight American soldiers was likely an accident, the U.S. military said Saturday, releasing the first results of an investigation into its worst ever loss of life here.

The soldiers died Thursday when a cache of mortar rounds and rifle ammunition discovered near Ghazni, about 90 miles southwest of Kabul (search), exploded.

Military spokesman Lt. Col. Bryan Hilferty said the soldiers were preparing the weapons for disposal when one or more of them detonated.

"There was a single pop and then a much louder, much larger sympathetic detonation," he said. "Initial indications are that it probably was an accident."

The status of an eighth soldier listed as missing was changed to "presumed deceased," Hilferty said Saturday, confirming the worse loss of life for U.S. forces since they invaded in late 2001 to oust the Taliban regime for harboring Usama bin Laden.

Hilferty gave no further details on the explosion, saying the investigation was still in its early stages. He had said Friday that the military was still looking into whether the cache was set off by a booby-trap.

The U.S. military has said the soldiers were on a regular patrol. But in the United States, a sister of one of the dead soldiers said officers informed relatives that the troops were on a training exercise.

Penny Cockerell, of Dallas, said she was told that her brother, Sgt. Kyle Seitsinger, a 29-year-old Army Reserve sergeant attached to the 10th Mountain Division (search), died when a mortar round exploded during the exercise.

Hilferty said several 10th Mountain soldiers were among the dead and that soldiers from other units also were killed. He declined to identify them further.

Afghan state TV broadcast a message of condolence from President Hamid Karzai (searchto President Bush on Friday evening, calling it "another sacrifice of your soldiers for peace and stability in Afghanistan."

Ghazni provincial Gov. Haji Asadullah Khan had said Friday that a U.S. patrol happened across an arms cache dating from the struggle against Soviet occupation in the 1980s and the ammunition "went off by accident."

The deaths came at the end of a month that has underlined the danger and instability still plaguing Afghanistan two years after the Taliban's ouster.

In January alone, about 80 people have died in violence in Afghanistan, including civilians, militants, police officers, international peacekeepers and now American soldiers.

Seitsinger, a former U.S. Marine, was attending university when called to active duty Dec. 1.

"There are so many loose weapons here and the wrong guys maintain control of them in clandestine areas," he wrote in a Jan. 14 e-mail to his sister. "It's our job to find and seize them."