The American soldier accused of killing 16 Afghan civilians, most of them children, had been trained as a sniper and recently suffered a head injury in Iraq, U.S. officials said Monday.

The name of the suspect, a married, 38-year-old father of two, has not been released.

U.S. Secretary of Defence Leon Panetta said the soldier may face capital charges, and that the U.S. must resist pressure from Washington and Kabul to change course in Afghanistan because of anti-American outrage over the shooting.

"We seem to get tested almost every other day with challenges that test our leadership and our commitment to the mission that we're involved in," Panetta told reporters travelling with him to Krygzystan. "War is hell."

A U.S. official said that during a recent tour of duty in Iraq, the suspect suffered a head injury in a traffic accident. The official spoke on condition of anonymity because the matter is under investigation. The vehicle accident was not a combat-related event, the official said.

There was no available indication about the extent of the injury, or whether his injury could be linked to any abnormal behaviour afterward.

Two U.S. officials speaking on condition of anonymity said the suspect had been trained as a sniper.

Rage boiling over

Sunday's attack in southern Kandahar province unfolded in two villages near a U.S. base. The shootings come as anti-Americanism already is boiling over in Afghanistan after U.S. troops burned Qur'ans last month and a video of marines urinating on alleged Taliban corpses was posted on the internet in January.

Panetta and U.S. President Barack Obama called Afghan President Hamid Karzai on Sunday to express their shock and sadness, and to offer condolences to the grieving families and the people of Afghanistan.

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An Afghan National Army soldier keeps watch inside a U.S. base in Panjwai district Kandahar province. Coalition forces have boosted security since 16 civilians were killed in a shooting spree by a U.S. soldier. (Ahmad Nadeem/Reuters)

The killing of Americans by their Afghan hosts and of Afghans by the Americans who are supposed to help them have forced an acute examination of a war strategy that calls for Afghans to assume greater responsibility for security through mentoring and "shoulder by shoulder" joint operations.

Obama said Monday the deadly incident will not trigger an early exit by American forces. However, he added, the massacre underscores the need to handover responsibility for security to Afghans and bring the troops home by 2014, he said.

"I think it's important for us to make sure that we get out in a responsible way so that we don't end up having to go back in," Obama told Pittsburgh station KDKA on Monday. "It makes me more determined to make sure that we're getting our troops home. It's time."

NATO and member countries said Monday the slayings were a blow to the alliance's efforts to cultivate trust, but would not affect the timeline to hand over security operations to Afghans by the end of 2014.

Taliban vows revenge

The U.S. president's comments come as the Taliban vowed revenge against U.S.-led forces for the weekend shooting spree. In a statement to the families of the victims, the Taliban pledged militants would avenge "every single martyr with the help of Allah."

The soldier, a staff sergeant who has been in the military for 11 years and served three tours in Iraq, was being held in pretrial confinement in Kandahar by the U.S. military while army officials review his complete deployment and medical history, Pentagon officials said.

The soldier's name was not released because it would be "inappropriate" to do so before charges are filed, said Pentagon spokesman George Little. But Panetta, his first public remarks on the incident, said Monday evening the death penalty is a consideration as the military moves to investigate and possibly put the suspect on trial.

U.S. troops brace for backlash

The promise of retaliatory violence against U.S.-led forces in Afghanistan put the U.S. Embassy on high alert as American residents in Kabul elected to remain in their homes due to fears of reprisals.

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An elderly Afghan man sits in a minibus on Sunday in Panjwai, Kandahar province next, to a covered body of a civilian allegedly killed by a U.S. service member. (Allauddin Khan/Associated Press)

"There's a wait-and-see attitude," freelance reporter Jennifer Glasse told CBC News from Kabul, adding that U.S. forces and the International Security and Assistance Force [ISAF] are watching the next two days with apprehension.

"All eyes really are on Kandahar, the southern city, where a prayer service tomorrow will be held at a main mosque. That will be really what everyone will be watching," Glasse reported.

She said elders in Kandahar had said there would be no attacks against U.S.-led forces, so long as they are satisfied the perpetrators behind Sunday's killings of mostly women and children are brought to justice.

U.S. officials said the suspected shooter, who turned himself in at his base, will be tried under U.S. law, indicating he won't be handed over to Afghan authorities.

Military movements were kept to a minimum Monday near the shooting site as commanders waited to see how the local population reacts, but there were no huge protests in the country. U.S. officials were worried that the Taliban would stoke public outrage this week in an attempt to turn the regular Friday prayer sessions into mass demonstrations.

Burning of Qur'ans led to initial violence

Frayed tensions in Afghanistan over the accidental burnings of Qur'ans at a U.S. army base quickly descended into bloody conflict last month. The Taliban claimed responsibility for several attacks, saying the desecration of the Islamic holy text by Americans warranted the violent reaction.

Afghan forces also turned their guns on their allies, killing six U.S. troops as violent protests racked the country. The weeks of violent protests and attacks also left some 30 Afghans dead, despite an apology from Obama.

Attacks 'deplorable'

Canadian Defence Minister Peter MacKay condemned the attacks as a "cowardly act of violence," and issued this statement on Sunday:

"I offer my most sincere condolences to the families and loved ones of those who have needlessly lost their lives. Today's shooting of Afghan civilians is deplorable and runs contrary to everything that the international mission to Afghanistan aims to accomplish."

Speaking at the UN on Monday, U.S. Secretary of State Hillary Clinton told reporters she was "shocked and saddened by the killings of innocent Afghan villagers" and sent her condolences to the victims' families.

"This not who we are," she said. "And the United States is committed to seeing that those responsible are held accountable."

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U.S. President Barack Obama talks with Afghanistan's President Hamid Karzai in this handout photograph taken and released on Sunday. Obama said he was deeply saddened by the killing of Afghan civilians. (Pete Souza/Reuters)

Kate Clark, a senior analyst with the Afghanistan Analysts Network, told CBC News from Kabul on Monday that a "grim atmosphere" could be felt on the ground there, as word of the massacre in two villages continued to spread.

"Either people are hearing about it on the radio or seeing the images on TV," she said. "We've had the Taliban vowing revenge; we've had Karzai saying this was an assassination and forgiveness is impossible; we've had the Afghan parliament also calling for justice."

Canadian legacy jeopardized

The weekend massacre could undo the fragile legacy of the Canadian army over its five-year Kandahar combat mission.

The Alaskan-based 3rd Battalion 21st U.S. Infantry Regiment — known as the Arctic Wolves — relieved Canadian troops in the troubled region last summer, allowing Ottawa to end the politically painful mission.

The Canadian military struggled to define its Afghan legacy as it left behind an unfinished war, one that cost billions of dollars and, at the time, 157 lives. One soldier has since died in the Kabul-centred training mission that took the place of combat operations. The army's Kandahar reputation was partly built on trust established with locals in Panjwaii, long known for their hostility to outsiders.

Defence experts say that although Canada has been gone for almost a year, like it or not the country's legacy is linked to the American massacre.

"Canada spent years of effort to create good will on behalf of [the International Security Assistance Force], and with the actions of one U.S. soldier, we're seeing all of that put at risk," said Walter Dorn, an associate professor and chair of international affairs at the Canadian Forces College.

Taliban condemn 'sick-minded American savages'

The Taliban said in a statement on their website that "sick-minded American savages" committed the "blood-soaked and inhumane crime" in two villages in Panjwaii district, a rural region outside Kandahar that is the cradle of the Taliban and where coalition forces have fought for control for years.

'This is an assassination, an intentional killing of innocent civilians and cannot be forgiven.'— Afghan President Hamid Karzai

The shooting suspect was deployed to Afghanistan on Dec. 3 with the 2nd Battalion, 3rd Infantry Regiment of the 3rd Stryker Brigade, 2nd Infantry Division, based at Joint Base Lewis-McChord located south of Seattle, a congressional source told The Associated Press.

He was assigned to help support a village stability operation in Belambai, about a kilometre away from one of the villages where the attack took place. Special operations troops pair with local residents chosen by village elders to become essentially a sanctioned, armed neighborhood watch.

The wire service cited an anonymous congressional source, whose name was withheld due to the sensitivity of the situation.

The killings were a blow to the community at Joint Base Lewis-McChord in Washington, a base which has seen its share of controversies and violence in the past few years.

Four Lewis-McChord soldiers were convicted in the deliberate thrill killings of three Afghan civilians in 2010. The base, home to about 100,000 military and civilian personnel, has also had a spate of suicides among soldiers back from war.

The army's criminal investigation division has started an investigation into the incident, said Chris Grey, a spokesman for the division. He declined to give other details to protect the integrity of the investigation.

Different directions

The U.S. military has said there is no indication that more than one soldier carried out the attacks in two villages in Kandahar province before dawn Sunday. But villagers told Afghan officials they heard shots being fired from several directions.

The Afghan Defence Ministry said the gunman left the base in Panjwai district and walked about 1,800 metres to Balandi village. Villagers described how they cowered in fear around 3 a.m. as gunshots rang out and the soldier roamed from house to house, firing on those inside. They said he entered three homes in all and set fire to some of the bodies after he killed them.

Eleven of the 12 civilians killed in Balandi were from the same family. The other victim was a neighbour.

From Balandi, the gunman walked roughly a kilometre to the village of Alkozai, which was only about 500 metres from the American military base. There the gunman killed four people in one house and then moved to another house, where he shot a man in the leg.

Some Afghan officials and villagers expressed doubt that a single soldier could have carried out the door-to-door shootings in the villages of Balandi and Alkozai, which lie about two kilometres apart, and burned the bodies afterward.  

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An Afghan man sits in the back of a bus with the body of a person who was allegedly killed by a U.S. service member in Panjwai, Kandahar province south of Kabul, Afghanistan, on Sunday. (Allauddin Khan/Associated Press)

"One man can't kill so many people. There must have been many people involved," Bacha Agha of Balandi village told The Associated Press. "If the government says this is just one person's act we will not accept it."

In a statement, Karzai also left open the possibility of more than one shooter, referring to "American forces" entering Afghan houses.

"This is an assassination, an intentional killing of innocent civilians and cannot be forgiven," Karzai said.

Lt. Col. Jimmie Cummings, another spokesman for the coalition, insisted there was only one gunman. "There's no indication that there was more than one shooter," he said.

With files from the CBC, The Canadian Press