Kabul, Afghanistan (CNN) -- The Afghan Taliban vowed Monday to exact revenge for the killing of 16 civilians, allegedly by an American soldier who went on a house-to-house shooting rampage Sunday in two villages near his base.
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 have a 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," Wali said. He asserted that Afghan security was "here to protect us, but (they) are protecting the Americans only."
"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 could intensify the rage that sparked deadly riots directed at international forces last month over the burning of Qurans by U.S. troops.
The soldier, an Army staff sergeant, turned himself in after shooting the 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.
"All evidence" indicates the suspect acted alone, an ISAF official said.
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, the military source said. A congressional source not authorized to speak publicly said he was deployed to the country in December and arrived at his current base in February.
The suspect is from Joint Base Lewis-McChord in Washington state, a military official said. The congressional source said the suspect is with the 3rd Stryker Brigade Combat Team, 2nd Infantry Division.
He worked in force protection at his outpost, and is a conventional army soldier supporting the Green Berets, according to a second military official who asked not to be named because of the investigation.
The probe is now being led by the Army's Criminal Investigation Command. The suspect was moved Monday from the outpost where he served -- identified by the congressional source as Village Stability Platform Belambai -- to detention in a larger U.S. location in Afghanistan, said the second military official, who declined to name the new location.
The villages were about 1 to 1.5 kilometers away from the combat outpost, the ISAF official said.
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.
"This is not who we are, and the United States is committed to seeing that those responsible are held accountable," U.S. Secretary of State Hillary Clinton said Monday.
But the 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.
"This is a very negative act in relations between the Afghans and the Americans," said a tribal elder in Panjwai who asked not to be named out of fears for his safety. "All Afghans have been hurt by this act and I don't think people will trust the Americans anymore."
"The Americans were telling the local people in Panjwai they should remain in their villages, and that 'we will help you and construct your schools, clinics and roads.' But in return the Americans went in and killed them," the elder said, adding he believes more than one soldier was involved.
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 of the incident, 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.
Obama released a statement saying the U.S. military will work to "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."
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
Officials within the Obama administration said the incident also will not derail talks on the role of U.S. troops beyond 2014, when foreign combat troops are scheduled to withdraw.
"This was a horrific and shocking incident," a senior administration official said. "But it does not change the strategic imperative for us to continue implementing our strategy -- defeating al Qaeda and strengthening the Afghan state so that groups like al Qaeda can never find a home there again."
The suspect will not face punishment under the Afghan justice system, said Pentagon spokesman George Little. "The U.S. military has strong means to address wrongdoing," he said. "There is an agreement in place with the government of Afghanistan, so that the investigation -- and when appropriate, prosecution -- will be done through U.S. military channels."
Indications are that the shootings were an "isolated incident," said State Department spokesman Mark Toner, who called it "horrific." The shootings, he said, do not "in any way reflect the values that we share with the Afghan people and our joint resolve to work together."
White House spokesman Jay Carney said Monday, "We believe that our presence (in Afghanistan) is having the desired effect in the implementation of the plan and achievement of our objectives." However, he acknowledged, "Incidents like this do not make it any easier. No question."
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.