The victims included nine children and three women. Most were shot in the head and their bodies were wrapped in blankets and burned.
At least five others were wounded. The 38-year-old Army staff sergeant from Joint Base Lewis-McChord in Washington state turned himself in.
Armed with at least two weapons, the rogue soldier walked south more than a mile and began stalking door to door. He entered one house wearing night vision goggles. In the dark, he killed four Afghans, then he shot a boy, who survived.
The shooting, which lasted less than 90 minutes, is under investigation.
It was the soldier's first deployment to Afghanistan, but he had been to Iraq three times before. The Pentagon is looking into the soldier's mental health history to see if he had been treated for post-traumatic stress.
The suspected shooter joined the Army after Sept. 11 and spent much of the last decade at war in Iraq as part of a brigade from Washington state.
Back home in 2008, the soldier went through a full mental health screening to become a sniper. He passed. At some point, according to ABC News, the soldier suffered a mild brain injury after hitting his head, but was treated successfully.
In December of 2011, he left for his first tour in Afghanistan.
The U.S. Soldier is a married father of two - his name has not been released. Secretary of defense leon panetta says the death penalty is a consideration as the military investigates. Afghan president hamid karzai says the intentional killing of innocent civilians cannot be forgiven -- and he's demanding an explanation from washington.
Secretary of State Hillary Rodham Clinton said the Obama administration regrets the killings.
"We recognize that an incident like this is inexplicable and will certainly cause many questions to be asked. But I hope that everyone understands in Afghanistan and around the world that the United States is committed to seeing Afghanistan continue its move toward a stable, secure, prosperous, democratic state," Clinton said.
The attack will likely spark even greater distrust between Washington and Kabul and fuel questions in both countries about why American troops are still fighting in Afghanistan after 10 years of conflict and the killing of Osama bin Laden.
U.S.-led forces in Afghanistan have stepped up security following the shootings Sunday in Kandahar province out of concern about retaliatory attacks. The Taliban has vowed revenge for the killings.
There is fear that the shooting will re-ignite the violent protests that swept across Afghanistan last month after U.S. servicemen burned Qurans.
The Taliban claimed responsibility for several attacks last month that the group said were retaliation for the Quran burnings. Afghan forces also turned their guns on their supposed allies, killing six U.S. troops as violent protests wracked the country.
The Associated Press contributed to this report.