The US soldier accused of killing 16 Afghan civilians could be executed if convicted, Leon Panetta, the US secretary of defence, has said.
Panetta told reporters that the shooting suspect would be brought to justice under the US military legal code, which allows for the death penalty in some cases.
The US army sergeant, who was on his first tour of duty in Afghanistan after serving three tours in Iraq, left his base in Kandahar province before dawn on Sunday and went on a killing spree, Afghan and US officials say.
He is accused of breaking into village homes and opening fire, killing 16 people including three women and nine children, in an incident that has imperilled Afghan-US relations anew after the burning of Qurans at a US military base.
"Then at some point after that (he) came back to the forward operating base and basically turned himself in and told individuals what happened," Panetta said late on Monday.
The Pentagon chief was asked if the suspect could be sentenced to death. "My understanding is in these instances that could be a consideration," he said.
Afghan President Hamid Karzai has described the shootings as "unforgivable," and the Afghan parliament declared that "people are running out of patience" over the behaviour of the 130,000 US-led NATO troops deployed in the country.
The Taliban has threatened to take revenge against "sick-minded American savages".
The Afghan parliament on Monday demanded that US officials "punish the culprits and try them in a public trial before the people of Afghanistan," then closed for the day in protest.
Al Jazeera’s Daniel Lak reporting from near Joint Base Lewis-McChord in Tacoma, Washington, where the soldier was deployed from, described the mood in the area as “shock”.
According to unofficial sources, Lak said, the soldier might have suffered minor brain trauma in a road accident during one of his three deployments in Iraq, but was cleared for deployment in Afghanistan last December.
Lak said that the soldier was sent to Kandahar in February as a sniper, however, he added that what details were available were being released in an “unofficial and unusual way and we’re still awaiting official confirmation".
No 'rush for the exits'
US President Barack Obama has warned against "a rush for the exits" amid mounting questions in Afghanistan about US war strategy.
In a series of interviews on Monday to local television stations, Obama said the massacre would not change strategy or plans for keeping troops in Afghanistan until 2014.
"It's important for us to make sure that we get out in responsible way, so that we don't end up having to go back in," Obama said. "But what we don't want to do, is to do it in a way that is just a rush for the exits."
"We have got hundreds of advisers in civilian areas as well, we have got huge amounts of equipment that have to be moved out. We have got to make sure that the Afghans can protect their borders to prevent al-Qaeda coming back."
Obama said Sunday's incident was not comparable to the 1968 My Lai massacre in Vietnam, when US troops killed hundreds of Vietnamese civilians in what was seen as a turning point in the US public's perception of the war.
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Just days before Sunday's attack, Kabul and Washington said they had made significant progress in negotiations on a strategic partnership agreement that would allow US advisers and special forces to stay in Afghanistan after foreign combat troops leave at the end of 2014.
But securing a full deal may be far more difficult now after the shooting spree in the southern province of Kandahar, the Taliban heartland.
"This could delay the signing of the strategic partnership agreement," an Afghan government official told the Reuters news agency.
Popular fury over the massacre, which brought demands that the United States withdraw earlier than scheduled, could be exploited by the Taliban to gain new recruits.
Anti-Americanism, which boiled over after copies of the Muslim holy book, the Quran, were inadvertently burned at a NATO base last month is likely to deepen after the Kandahar carnage.
"The Americans said they will leave in 2014. They should leave now so we can live in peace," said Mohammad Fahim, 19, a university student. "Even if the Taliban return to power our elders can work things out with them. The Americans are disrespectful."