ABC News has learned that the soldier suffered a mild traumatic brain injury in 2010 while on deployment in Iraq.
It is unclear if the injury could have played a role in Sunday's massacre. An official told ABC News that the soldier has suffered a mild traumatic brain injury (TBI) in the past, either from hitting his head on the hatch of a vehicle or in a car accident. He went through the advanced TBI treatment at Fort Lewis and was deemed to be fine.
He also underwent mental health screening necessary to become a sniper and passed in 2008. He had routine behavioral health screening after that and was cleared, the official said.
When the soldier returned from his last deployment in Iraq he had difficulty reintegrating, including marital problems, the source told ABC News. But officials concluded that he had worked through those issues before deploying to Afghanistan.
Investigators are also looking at the possibility that alcohol may have played a role in the incident, defense officials said, as alcohol was found near where he lived on base in Afghanistan.
On Monday, the suspect appeared before a magistrate where probable cause for continued pre-trial confinement was established.