(CBS News) WASHINGTON - U.S. officials are still investigating the suspect in Sunday's massacre of 16 Afghan civilians -- and apologizing for it.
The Army hasn't brought charges yet against the sergeant, who the Pentagon still refuses to identify, but late Monday, Defense Secretary Leon Panetta said prosecutors could seek the death penalty.
On a trip to Asia and the Middle East, Panetta seemed resigned to the fact that, as long as U.S. troops are in Afghanistan, bad things are bound to happen.
"War is hell," said the defense chief. "These kinds of events and incidents are going to take place. They've taken place in any war; they're terrible events. And this is not the first of those events, and it probably won't be the last."
The motive for the shootings is still unknown. Investigators will pour over the personal documents and writings of the 38-year-old sergeant, searching for evidence of his state of mind -- did he set out to kill when he left the base?
Investigators will comb "written letters, e-mails, all his accounts," military defense lawyer Neal Puckett tells CBS News. "I'm sure all that's been seized and locked down and (they're) going to be searched for any hint or any indication. Facebook pages often are the source of information about what people are thinking."
Eyewitnesses claim they saw the U.S. soldier drag young boys by the hair and shoot them in the mouth. Some of the victims were as young as two.
Puckett thinks investigators should be able to match any spent shells recovered at the scene with the sergeant's weapon.
"Assuming that bullets are recovered and the rifle the individual is alleged to have used is available, I would think that would be a very simple task for them," he notes.
Four years ago, the sergeant passed a battery of psychological tests in order to qualify as a sniper. Then, in 2010, he was evaluated for mild traumatic brain injury after rolling his vehicle in Iraq, but he was judged fit to return to duty.
He was stationed at Joint Base Lewis-McChord in Washington state, but had served three tours in Iraq and was on his first deployment to Afghanistan, assigned to a special operations unit training villagers in southern Afghanistan in how to fend for themselves.
The sergeant is married with two children. For their own safety, his family has been moved out of their home and onto Joint Base Lewis-McChord.
To see David Martin's report, click on the video in the player above.