WASHINGTON -- The alleged massacre of Afghan civilians by a U.S. soldier is "outrageous and unacceptable," President Barack Obama said yesterday, and he promised a thorough and unstinting Pentagon investigation.

"The United States takes this as seriously as if it was our own citizens, and our children, who were murdered. We're heartbroken over the loss of innocent life," Obama said. He sounded stern and emotional in brief remarks on the weekend killings, made before an unrelated White House event.

"I've directed the Pentagon to make sure that we spare no effort in conducting a full investigation," Obama said. "We will follow the facts wherever they lead us, and we will make sure that anybody who is involved is held fully accountable with the full force of the law."

Defense Secretary Leon Panetta has said the death penalty could be possible in the case.

Obama's message was aimed at Afghans and at Americans, for whom the killings were a reminder that tens of thousands of U.S. forces are fighting in Afghanistan more than 10 years after the war began. He insisted the killings will not change the U.S. commitment to finishing the job in Afghanistan, but he was clearly trying to reassure Americans he would close out the war. "Make no mistake, we have a strategy that will allow us to responsibly wind down this war," Obama said.

Also yesterday:

Taliban insurgents opened fire on two brothers of Afghan President Hamid Karzai as they left a memorial service for the 16 villagers allegedly killed by a U.S. soldier. Qayum and Shah Wali Karzai and other top Afghan officials escaped in their cars unharmed from the ambush in the country's south. An Afghan soldier was hit in the head almost immediately and died; two other Afghan army personnel were wounded in the 20- minute firefight that ensued in one of the two villages in Kandahar province where the killings had occurred two days before.

A military court held a hearing for the American soldier suspected of killing the Afghan civilians and found probable cause to keep him in detention, a spokesman said. The soldier, who has not been named, is accused of leaving a U.S. base in Kandahar and gunning down the civilians, including nine children and three women, in two villages.

Panetta said he was awaiting details from Gen. John Allen, top commander in Afghanistan, on his plan for bringing home the remaining 23,000 troops sent to Afghanistan during the 2010 surge.

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