The soldier's name has not been released, but a U.S. official told ABC News he is a 38-year-old staff sergeant who is married with two children, and served three tours in Iraq. This was his first tour in Afghanistan, where he has been since early December, the official said.
He is accused of opening fire on villagers near his base in southern Afghanistan Sunday and killing 16 civilians, according to President Hamid Karzai, who called it an "assassination" and furiously demanded an explanation from Washington.
Nine of the victims were children, and three were women, all shot while they slept in their beds, according to villagers and the Afghan president's office.
NATO's International Security Assistance Force said the soldier acted alone and turned himself in to his commanders after opening fire on civilians. U.S. President Barack Obama called the killings "tragic and shocking," and offered his condolences to the Afghan people.
The soldier remains in NATO custody.
A U.S. official told the Associated Press that the staff sergeant was a conventional soldier assigned to support a special operations unit of either Green Berets or Navy SEALs engaged in a village stability operation.
The officials spoke on condition of anonymity because the investigation is still ongoing.
The shooting took place at 3 a.m. in two villages in the Panjwai district of southern Kandahar province, a hotbed for the Taliban insurgency against U.S. forces. The two villages are a short walk away from the U.S. base where the soldier was stationed.
Photos from the scene show blood-splattered floors and walls inside a villager's home, one of three believed to have been attacked, and blood-soaked bodies of victims, including the elderly and young children, wrapped in blankets and placed in the backseat of a van.
Some of the bodies appear to have been burned, and some witnesses said the soldier set fire to some of his victims after shooting them.
Lewis-McChord personnel have been implicated before in other violence in the same area.
Four soldiers from a Stryker brigade out of Lewis-McChord have been sent to prison in connection with the 2010 killing of three unarmed men during patrols in Kandahar province's Maiwand district, which is just northwest of Panjwai.
They were accused of forming a "kill team" that murdered Afghan civilians for sport - slaughtering victims with grenades and powerful machine guns during patrols, then dropping weapons near their bodies to make them appear to have been combatants.
Jorge Gonzalez, executive director of a veterans resource center near Joint Base Lewis-Mcchord named GI Voice-Coffee Strong, said Sunday's killings offer more proof that the base is dysfunctional.
"This was not a rogue soldier. JBLM is a rogue base, with a severe leadership problem," he said in a statement.
He cites the "kill team" incident, multiple suicides among base personnel and spousal abuse by soldiers as more symptoms of problems at the base.
The military newspaper Stars and Stripes in 2010 called JBLM the "most troubled base in the military," quoting retired Army Gen. Barry McCaffrey on the base's "severe leadership problem."
A Los Angeles Times article in 2011 called JBLM a "base on the brink" after a wave of suicides among soldiers sent on repeated deployments.
Lewis-McChord, a sprawling complex of red brick buildings, training fields and forests, is about 45 miles south of Seattle and has grown quickly since the Sept. 11, 2001, terror attacks.
Officials there have said that any community the size of the base is bound to have its problems, and its reputation has been tarred by "a small number of highly visible but isolated episodes" that don't accurately reflect the remarkable accomplishments of its service members, including their work overseas and the creation of new programs to support returning soldiers.