KABUL, Afghanistan (CNN) -- Afghans railed at American forces Monday as investigators combed through two villages where a U.S. Army sergeant is accused of having gone on a weekend rampage, killing 16 civilians.
Men wept openly as they showed the bodies of their neighbors -- and some of their neighbors' children -- on Monday. In one truck, a toddler with a bloodstained face lay between the bodies of two men, while another held the charred remains of two more people.
"One guy came in and pulled a boy from his sleep and he shot him in this doorway. Then they came back inside the room and put a gun in the mouth of one child and stomped on another child," one woman said.
"This base told us to come back to our villages," another woman shouted. "They said, 'We won't bother you, this is your land and this is your own village.' Then those dogs come and grab us?"
Some of the villagers said more than one soldier appeared to have been involved in the killings. But Gen. John Allen, the commander of NATO's International Security Assistance Force, said investigators believe the suspect, a veteran of three previous tours of duty in Iraq, "acted as an individual."
The dead included four men, three women and nine children, Afghan officials reported.
The suspect was based at a combat outpost near the villages. His motive remained unknown Monday. He invoked his right to remain silent after turning himself in to his comrades early Sunday, according to a senior Defense Department official, who spoke on condition of anonymity.
The official said the suspect was an infantry sniper from Joint Base Lewis-McChord, outside Tacoma, Washington. He was diagnosed with a traumatic brain injury in 2010, after a vehicle rollover in Iraq, but found fit for duty after treatment, the official said.
Other officials have identified the soldier as a staff sergeant in his mid-30s, with a wife and children and 11 years of service in the Army. In an interview on CNN's "The Situation Room," Allen said the suspect's medical history would be part of the investigation, which would be handled by American military authorities.
Sunday's killings followed a string of incidents involving U.S. troops that have strained ties between the United States and Afghanistan, and Afghan President Hamid Karzai condemned the weekend bloodshed as "unforgivable." Afghanistan's parliament demanded a public trial for the suspect, while the Afghan Taliban condemned U.S. troops as as "sick-minded American savages" and vowed to exact revenge.
But Allen said the decade-old allied campaign in Afghanistan "remains on track" despite the violence.
"Every single day, Afghan soldiers and Afghan police and ISAF troops are serving shoulder-to-shoulder in some very difficult situations," he said. "And our engagement with them, our shoulder-to-shoulder relationship with them, our conduct of operations with them every single day defines the real relationship. This is an isolated act."
But people in the area where the killings took place 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 said 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 suspect had been deployed to Afghanistan around the beginning of the year and arrived at his current base in February, according to a congressional source not authorized to speak publicly. The congressional source said the suspect is with the 3rd Stryker Brigade Combat Team, 2nd Infantry Division, which is stationed at Joint Base Lewis-McChord.
He worked in force protection at his outpost, and is a conventional army soldier supporting the Green Berets, according to a military official who asked not to be named because of the investigation.
The investigation is 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 military official, who declined to name the new location.
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.
"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.
Afghan troops spotted the soldier leaving his combat outpost around 3 a.m. Sunday and notified their American counterparts, according to ISAF. 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."
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.
Taliban link attack to Quran burning
Officials within the Obama administration said the incident 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, after 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.
CNN's Sara Sidner, John Dear, Jethro Mullen, Barbara Starr, Chris Lawrence, Brianna Keilar, Diana Magnay, Deirdre Walsh and journalist Ruhullah Khapalwak contributed to this report.