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."
The incident has plunged US-Afghan relations to their lowest point and threatens US negotiations with the Afghan government over the development of a long-term security relationship.
It has added to anger over the burning of Muslim holy books by US forces on a base in Afghanistan earlier this year, which sparked violent protests and attacks.
The US embassy in Kabul has issued an emergency warning and called for vigilance amid fears of anti-US protests and possible revenge attacks in the coming days.
Stuart Ramsay, reporting from Kabul, said: "The Afghan government is absolutely furious about this and there are expectations of protests to come.
"The actual population is furious as well and that in many ways is worse because it breaks down all the work that it being done in the communities so that Isaf and British forces can leave over the next few years."
Taliban militants have already vowed to hit back against "the invaders and the savage murderers for every single martyr" in a statement posted on their website.
It said: "A large number from among the victims are innocent children, women and the elderly, martyred by the American barbarians who mercilessly robbed them of their precious lives and drenched their hands with their innocent blood."
Sky News foreign affairs editor Tim Marshall added: "The Americans are on high alert for reprisals and there will be reprisals, and the Afghan government will now use this as leverage ahead of the planned troop withdrawal."
At a news conference on Monday, Isaf spokesman Brigadier General Carsten Jacobson admitted the incident may derail wider coalition plans.
He said: "From Isaf's point of view these incidents, and in particular the short distances between the incidents, are a burden and of concern for Isaf."
But the White House is insistent its strategy will not waver and that plans to hand back control of security to Afghan troops at the end of 2014 will not be affected.
Spokesman Jay Carney said: "The focus of our overall strategy is not in reaction to a single event. I do not believe that this incident will change the timetable of a strategy that was designed and is being implemented in a way to allow for the withdrawal of US forces, to allow for the transfer of lead security authority over to the Afghans."
US Secretary of State Hillary Clinton added: "This terrible incident does not change our steadfast dedication to protecting the Afghan people and to doing everything we can build a strong, stable Afghanistan."
She also expressed her shock and sadness at the shooting, and stressed: "This is not who we are and the United States is committed to seeing that those who are responsible are held accountable."
Prime Minister David Cameron, who is visiting Mr Obama in the US this week, also insisted the incident must not be allowed to "derail the very good job done by US, UK and Isaf troops".
And Foreign Secretary William Hague told Sky News: "The (deaths) do not change the overall position on Afghanistan and our determination to safeguard our own national security, or to hand over control during the next couple of years to Afghanistan nation security forces.
"We've done a lot of that already, to make sure Afghanistan cannot be a danger to the rest of the world in the future."