In the dark of night, a U.S. soldier left his base in southern Afghanistan's Kandahar province, walked a short distance to two villages, entered the homes of Afghan civilians and opened fire killing 16 people, including nine children.
The rampage, in the province long patrolled by Canadian troops, is rocking the strained relationship between Western forces and their Afghan hosts, already raw since the burning of Muslim holy books on a base last month sparked weeks of protests that left 30 dead.
Reports from the rural area where the murders took place described them as methodical, with the American Army staff sergeant trying door after door and finally entering three homes. He reportedly set fire to the first, after killing 11 members of the same family, including four girls younger than six.
"This is an assassination, an intentional killing of innocent civilians and cannot be forgiven," Afghan President Hamid Karzai said in a statement on Sunday, as news of the killings spread along with graphic pictures of the dead. Mr. Karzai also repeated his complaint that the U.S.-led NATO forces in his country for the last 10 years conduct operations that sometimes kill Afghan civilians.
Condolences and pledges of investigations poured from U.S. President Barack Obama, NATO military commanders and political leaders among allies, including Canada's Defence Minister, Peter Mackay.
But they were clearly holding their breath for the answer to another question: Will Afghans, living for years with a vast Western military presence, accept that responsibility lies with a lone rogue soldier?
The killings took place in a district familiar to Canadian troops: in villages in Panjwaii district, the rural heartland of the Taliban in Kandahar province, where the Canadian Forces patrolled from 2006 to 2011, fighting a hide-and-seek war with insurgents.
It's is now the scene of a new tragedy, one threatening to set alight new tensions at a particularly delicate moment in relations between Afghanistan and the foreign troops and aid workers who came to fight the Taliban.
In Canada, Mr. Mackay issued a statement in which he called the attack a "cowardly act" and said the shooting "runs contrary to everything that the international mission to Afghanistan aims to accomplish."
U.S. officials worked to limit the damage, stressing the killings will be investigated and punished. Mr. Obama, who phoned Mr. Karzai to offer condolences, issued a statement calling the attack "tragic and shocking" and not representative of the U.S. military. He vowed to "get the facts as quickly as possible and to hold accountable anyone responsible."
But it's not clear if those pledges of justice will ally Afghan outrage, already fuelled by Taliban claims of a NATO-backed massacre, photographs of dead bodies posted on social media sites, and confusion about how such an attack could happen.
The New York Times reported that a senior American military official confirmed that the sergeant was attached to a unit based at Fort Lewis, near Tacoma, Wash., and that he had been part of what is called a village stabilization operation in Afghanistan, in which teams of Green Berets, supported by other soldiers, try to develop close ties with village elders, organize local police units and track down Taliban leaders. The official said the sergeant was not a Green Beret himself, and had been deployed to either Iraq or Afghanistan at least once before his current tour of duty.
Many Afghans were already questioning whether the killings were really the work of one soldier, just as they questioned the explanation from NATO commanders earlier this month that the burning of Korans at the Bagram airbase was inadvertent. Six U.S. service members have been killed by their Afghan colleagues since the burnings came to light, and the violence had just started to calm down.
Even among senior Afghan officials, explanations that a lone soldier went rogue left pointed questions, according to an Afghan reporter for the BBC, Bilal Sarwary.
"They said it appears he had a mental problem, he had a breakdown," Mr. Sarwary said in a telephone interview from Kabul. "But officials here in Kabul, including presidential aides, are now asking, 'How could he walk out of a base?' "
He said he was told that aides to Mr. Karzai decided to delay telling him of the killings until after he delivered an unrelated speech on Sunday because they feared his response would be provocative.
The killings could shake the already fragile trust between Western forces and Afghans – at a time when the United States and other allies are considering the pace for a planned transfer of control to Afghan forces.
"This is a fatal hammer blow on the U.S. military mission in Afghanistan. Whatever sliver of trust and credibility we might have had following the burnings of the Koran is now gone," said David Cortright, director of policy studies at Notre Dame's Kroc Institute for International Peace Studies and an advocate for a quick withdrawal from Afghanistan.
"This may have been the act of a lone, deranged soldier," Mr. Cortright added in an interview with The Associated Press. "But the people of Afghanistan will see it for what it was, a wanton massacre of innocent civilians."
The attack, which started at about 3 a.m., took place in two villages in the Panjwaii district, Balandi and Alkozai, about 500 metres from a U.S. base.
Twelve of the dead were from Balandi, said Samad Khan, a farmer who lost 11 members of his family, including women and children. He told the AP he was away from the village and returned to find his family members shot and burned. A neighbour was also killed.
Mr. Khan and other villagers demanded that Mr. Karzai punish the American shooter. "Otherwise we will make a decision," he said. "He should be handed over to us."
The four people killed in Alkozai were all from one family, said a female relative who was shouting in anger. "No Taliban were here. No gun battle was going on," she said. "We don't know why this foreign soldier came and killed our innocent family members. Either he was drunk or he was enjoying killing civilians."
Ehsanullah Ehsan, a school director in Kandahar who runs the Afghan-Canadian Community Centre, said by telephone that people in Kandahar City were calm, but saddened.
"This might incite certain uprisings or things like that, but we still don't know what will happen," he said. "Will they just bury their dead, or start demonstrations and protests? We don't know yet."
With reports from Graeme Smith, The New York Times and Associated Press