Taliban vows revenge for U.S. soldier's shooting rampage The Afghan Taliban criticizes "sick minded American savages" It vows to exact revenge for the killing of 16 people by an American soldier The soldier is now in custody as investigators seek to establish what motivated him The attack has fueled concerns of fresh unrest following recent riots over Quran burnings Kabul, Afghanistan (CNN) -- The Afghan Taliban said Monday that its fighters would exact revenge for 16 people left dead after an American soldier went on a house-to-house shooting rampage in two villages a day earlier. Describing U.S. forces as "sick minded American savages," the Taliban said in a statement on its website that it would mete out punishment for the "barbaric actions." The Taliban, an Islamic fundamentalist movement, has battled the U.S.-led coalition in Afghanistan for a decade. U.S. officials have expressed shock and sadness over the attack, while Afghan leaders have angrily condemned it. President Hamid Karzai of Afghanistan called it an "unforgivable" crime, noting that nine of the dead were children. The killings have fueled fears of intensified ire directed at international forces in the country following deadly riots over the burning of Qurans by U.S. troops late last month. 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 now in U.S. custody as investigators try to establish what motivated him. The attacker's mental stability and medical history are among "the things the investigators are looking at," said Capt. John Kirby, an ISAF spokesman. This was a soldier who had been in the army some time, had deployed before. Kirby said. This the wasn't his first deployment. But with respect to specific motives, we just can't say right now. U.S. President Barack Obama called the killings "tragic and shocking," and offered his condolences to the Afghan people in a phone call to Karzai, the White House said. But his comments appeared unlikely to soothe the outrage among Afghans. "The Afghan people can withstand a lot of pain," said Prince Ali Seraj, the head of the National Coalition for Dialogue with the Tribes of Afghanistan. The killings took place in the district of Panjwai, about 25 km (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. The wounded Afghans were being treated in one of ISAF's facilities. A U.S. military official said that the suspect is from Joint Base Lewis-McChord in Washington state. Afghan troops spotted the soldier leaving his combat outpost around 3 a.m. Sunday and notified their American counterparts, according to ISAF. The U.S. military did an immediate headcount, found the soldier was missing and dispatched a patrol to go look for him. He said the attack "does not represent the exceptional character of our military and the respect that the United States has for the people of Afghanistan." But Seraj, a member of Afghanistan's former royal family, said the killings are likely to play into the hands of the Taliban. Kirby said that although the attack Sunday was "very, very tragic," it wasn't "having a major effect across the country with respect to the mission our troops are doing every day." Amid the uproar over the attack, Chancellor Angela Merkel of Germany arrived in Afghanistan on Monday. Merkel is spending a day visiting German troops in the northern Afghan city of Mazar-e-Sharif, according to the German government's press office. CNN's Sara Sidner, John Dear and Jethro Mullen contributed to this report.