The US soldier accused of killing 16 villagers in Afghanistan had suffered a traumatic brain injury on an earlier tour in Iraq, according to a US official.
The army staff sergeant was seriously injured when he was in a vehicle that rolled during a tour of duty in 2010, the official revealed.
Speaking on condition of anonymity, the source said it was too early to say whether there was any link between the injury and the shooting in Afghanistan.
Fresh information about the alleged gunman emerged as US President Barack Obama insisted the incident should not be allowed to overshadow the wider Afghan mission.
"Obviously what happened this weekend was absolutely tragic and heartbreaking but when you look at what hundreds of thousands of our military personnel have achieved under enormous strain, you cannot help but be proud generally," he said.
"I think it is important for us just to make sure that we are not in Afghanistan longer than we need to be."
The alleged gunman, who apparently returned to the base and turned himself in following the shooting, is a 38-year-old staff sergeant who is married with two children, a US official told ABC News.
He is believed to have deployed to Afghanistan for what was his first tour in the country from the US army and air force's Joint Base Lewis-McChord near Tacoma in Washington state and had previously served on three tours of Iraq.
Sky's chief correspondent Stuart Ramsay said he was involved in "village stabilisation operations", in which army personnel support special forces Green Berets.
Ramsay said: "Their job is to create, in effect, an armed neighbourhood watch that work with trusted village elders."
The Pentagon has said it will not identify the soldier until charges are filed.
The Afghan parliament has demanded that he face a public trial over the shootings on Sunday.
In a statement, the lower house of parliament called the killings "brutal and inhuman" and declared "people are running out of patience over the ignorance of foreign forces".
"We seriously demand and expect that the government of the United States punish the culprits and try them in a public trial before the people of Afghanistan," it said.
The soldier allegedly opened fire on civilians during a rampage in two villages around 500m from a US base in the southern province of Kandahar.
The dead included nine children and three women. It has been claimed 11 were shot down in one house and included four girls under the age of six.
Afghan President Hamid Karzai has demanded an explanation for what he called an "assassination" that "cannot be forgiven".
The US has pledged to hold a full investigation and punish those responsible but the Pentagon has rejected calls for a trial in Afghanistan.
Spokesman George Little said there were agreements in place with the Afghan administration governing the prosecution of US troops serving in the country.
He stressed that "the United States military has very strong means to address wrongdoing", adding: "We believe that this was a lone US servicemember who was involved in this deeply tragic set of shootings. This is an isolated incident and we will pursue accountability for the alleged actions of this servicemember."
