Many details remain unknown about Sunday's shooting in southern Afghanistan, where a U.S. Army sergeant is suspected of walking through villages near Kandahar and killing 16 Afghan civilians.
But the shooting has raised the specter of reprisals against American troops and also led to questions about how much damage it could cause to the larger American war effort in Afghanistan.
Here's a look at what is, and isn't, known so far.
U.S. officials have not released the name of the soldier suspected in the shooting. But the military has identified him as a 38-year-old staff sergeant, married and a father of two.
Officials say this is his first tour in Afghanistan after multiple deployments in Iraq. He was based in Fort Lewis, Wash., and belongs to a unit supporting Special Forces who are working with the Afghan police force.
U.S. officials say the soldier surrendered to authorities and remains in U.S. custody, but they have not offered any possible motivation for the attack.
Some villagers in the rural Panjwayi district, the site of the slayings, told NPR's Quil Lawrence that more than one American soldier was involved.
However, U.S. officials say no combat operations were taking place in the region at the time of the killings and that they have no evidence that other soldiers were involved.
"One [shooting] was a quarter of a mile outside the American base and the other in a hamlet about a mile away," Lawrence reports from Kabul. "In the first village, about 11 people died in one family, most of them young children in one house. Their bodies were apparently gathered up under some blankets, which were then set alight."
The New York Times reports that some villagers say the soldier "tried door after door, eventually breaking in to kill within three separate houses."
The U.S. Response
President Obama and Defense Secretary Leon E. Panetta each have called Afghan President Hamid Karzai and expressed condolences, promising thorough investigations.
"This incident is tragic and shocking, and does not represent the exceptional character of our military and the respect that the United States has for the people of Afghanistan," the president said in a statement.
The Taliban, meanwhile, has vowed vengeance for the shootings, calling them a "blood-soaked and inhumane crime" by "sick-minded American savages," according to The Associated Press.
A Troubled Base
The sergeant suspected in the shootings Sunday deployed from the Joint Base Lewis-McChord in Washington state, one of the largest bases in the U.S. The Lewis-McChord installation has become a lightning rod for criticism about military assistance for soldiers transitioning from war zones.
Last year, Lewis-McChord reported a base record of 12 suicides, according to the AP. Four Lewis-McChord soldiers were convicted in one of the worst incidents in the war — the 2010 killing of three Afghan civilians for sport, which occurred in the same region.
Threat To U.S. Mission
The attacks Sunday come after U.S. troops last month burned copies of the Quran, apparently inadvertently. In another recent incident that drew widespread attention, American Marines were captured on video urinating on the corpses of militants. The Quran-burning led to deadly protests.
All of these episodes could hamper U.S. efforts to train Afghan forces and arrange peace talks with the Taliban as part of the American plan to wind down its role in Afghanistan after more than a decade of war.
"Counterinsurgencies are all about winning over the people and getting a government to work with the U.S.," NPR Pentagon correspondent Tom Bowman says. "This clearly doesn't help matters."