The American soldier suspected of killing 16 Afghan civilians on Sunday is in his mid-30s and served several tours of duty in Iraq, but he is on his first deployment to Afghanistan, a U.S. military official said Monday. The official asked not to be named discussing an ongoing investigation.

The soldier, an Army staff sergeant, acted alone and turned himself in after opening fire on civilians, according to officials from NATO's International Security Assistance Force, or ISAF. He is in U.S. custody as investigators try to establish what motivated him.

The killings took place in the district of Panjwai, about 25 kilometers (15 miles) southwest of Kandahar, southern Afghanistan's major city, according to Karzai's office. The dead included four men, three women and nine children, it said, while five people were wounded.