President Barack Obama and Defense Secretary Leon Panetta are expressing their condolences to the people of Afghanistan.
Both men called Afghan President Hamid Karzai after an American soldier in Afghanistan wandered off base and allegedly gunned down 16 villagers.
In a statement released Sunday by the White House, Obama called the incident "tragic and shocking."
Panetta says a full investigation is already under way.
A U.S. official says the suspect is a conventional soldier from Joint Base Lewis-McChord, Wash. He was assigned to support a special operations unit of either Green Berets or Navy SEALs engaged in a village stability operation.