Afghanistan’s parliament, meanwhile, demanded a public trial for the suspect, who is accused of killing nine children, three women and four men.

“We strongly request the government of America to punish this wild act and a have public trial in front of the people of Afghanistan,” lawmakers said in a statement Monday.

The Taliban called U.S. forces “sick-minded American savages,” warning in an online statement that the group would mete out punishment for the “barbaric actions.”

U.S. officials have expressed shock and sadness over the attack. Afghan leaders have angrily condemned it. President Hamid Karzai of Afghanistan called the attack an “unforgivable” crime.

People in the area of the killings are angry at both Americans and Afghan security forces, whom they accuse of failing to protect them, villager Muhammad Wali said.

“Villagers were cursing at them,” and saying Afghan security was “here to protect us, but you are protecting the Americans only,” Wali said.

“The people in these villages are scared, and we don’t know what is going to happen next. … They saw nothing except the Americans going and killing them in their homes,” he said.

The killings have fueled fears of intensified ire directed at international forces in the country following deadly riots last month over the burning of Qurans by U.S. troops.

The soldier, an Army staff sergeant, acted alone and turned himself in after opening fire on civilians, according to officials from NATO’s International Security Assistance Force, or ISAF. He is in U.S. custody as investigators try to establish what motivated him.

He is in his mid-30s and has served several tours in Iraq, but he is on his first deployment to Afghanistan, said a U.S. military official, who asked not to be named talking about an ongoing investigation.

He arrived in Afghanistan in January and was supporting the Green Berets, the source said.

The suspect is from Joint Base Lewis-McChord in Washington state, a military official said, adding the soldier is assigned to a Special Forces unit.

The probe is now being led by the Army’s Criminal Investigation Command.

The attacker’s mental stability and medical history are among “the things the investigators are looking at,” said Capt. John Kirby, a spokesman for the NATO-led force.

“This was a soldier who had been in the Army some time, had deployed before.” Kirby said. “This wasn’t his first deployment. But with respect to specific motives, we just can’t say right now.”

U.S. President Barack Obama called the killings “tragic and shocking” and offered his condolences to the Afghan people in a phone call to Karzai, the White House said.

But his comments appeared unlikely to soothe the outrage among Afghans.

“The Afghan people can withstand a lot of pain,” said Prince Ali Seraj, the head of the National Coalition for Dialogue with the Tribes of Afghanistan. “They can withstand collateral damage. They can withstand night raids. But murder is something that they totally abhor, and when that happens, they really want justice.”

The killings took place in the district of Panjwai, about 25 kilometers (15 miles) southwest of Kandahar, southern Afghanistan’s major city, according to Karzai’s office. The dead included four men, three women and nine children, it said, while five people were wounded.

The wounded Afghans were being treated in one of the NATO-led force’s facilities. The allied command did not give its estimate of casualties.

There were no military operations in the area, either on the ground or in the air, at the time, according to two senior coalition officials who spoke on condition of anonymity because of the sensitivity of the situation.

Afghan troops spotted the soldier leaving his combat outpost around 3 a.m. Sunday and notified their American counterparts, according to the NATO-led force. The U.S. military did an immediate headcount, found the soldier was missing and dispatched a patrol to go look for him. The patrol met him as he returned and took him into custody.

In a statement, Obama said the U.S. military will “get the facts as quickly as possible and to hold accountable anyone responsible.”

White House response to shooting spree

He said the attack “does not represent the exceptional character of our military and the respect that the United States has for the people of Afghanistan.”

In a separate statement, Defense Secretary Leon Panetta said he was “shocked and saddened” by the attack and said the suspect was “clearly acting outside his chain of command.”

But Seraj, a member of Afghanistan’s former royal family, said the killings are likely to play into the hands of the Taliban.

“They are really going to milk this for all it’s worth,” Seraj said. “This is playing right into their program of psychological warfare against the Afghan people.”

Seraj called for a joint U.S.-Afghan investigation into the killings, saying Afghans will want to see “quick and decisive justice.”

Kandahar and the surrounding region is the home of the Taliban, and eight of the 69 coalition troops killed in Afghanistan so far this year died in the province.

Kirby said that although the attack Sunday was “very, very tragic,” it wasn’t “having a major effect across the country with respect to the mission our troops are doing every day.”

Taliban link attack to Quran burning

The United States and its allies invaded Afghanistan in 2001, following al Qaeda’s attacks on New York and Washington that killed nearly 3,000 people. The invasion quickly toppled the Taliban, an Islamic fundamentalist movement that ruled most of Afghanistan and had allowed al Qaeda to operate from its territory. But the militia soon regrouped and launched an insurgent campaign against the allied forces and a new government led by Karzai.

The No. 1 U.S. target in the conflict, al Qaeda founder Osama bin Laden, was killed in a commando raid in neighboring Pakistan in May 2011. American and allied combat troops are scheduled to leave Afghanistan by 2014, and Karzai has been increasingly critical of the allied force.

Tensions ramped up dramatically in February after a group of U.S. soldiers burned copies of the Quran, Islam’s holy book, that had been seized from inmates at the American-run prison at Bagram Air Base because they allegedly contained extremist communications.

American officials from Obama down called the burning an accident and apologized for it, but riots left dozens dead, including six American troops. Hundreds more Afghans were wounded.

Amid the uproar over the attack, Chancellor Angela Merkel of Germany arrived in Afghanistan on Monday and called Karzai to express her condolences and those of the German people over the attack, the German government press office said.

She assured Karzai that the NATO-led force would do everything in its power to establish the circumstances of the attack.

Visiting German troops in the northern Afghan city of Mazar-e-Sharif, she said a planned German pullout in 2014 was not certain, the government said. The German parliament set that date for withdrawal in a vote in January 2011.

Merkel had intended fly into Kunduz, but bad weather prevented her from doing so.