Fort Hood gunman Ivan Lopez 'may have snapped after row'
The US Army says there is a "strong possibility" a gunman who killed three people at a base in Texas was involved in an argument before the shooting.
Lt Gen Mark Milley formally identified the attacker as Specialist Ivan Lopez, 34, who later shot himself.
He told reporters there was "very strong evidence" that Lopez, originally from Puerto Rico, had an unstable psychological condition.
Sixteen others were wounded in the attack.
Officials have said that Lopez had served in Iraq and was being assessed for post-traumatic stress disorder.
"There may have been a verbal altercation with another soldier or soldiers," Gen Milley said.
"And there's a strong possibility that that in fact immediately preceded the shooting, but we do not have that definitively at this point, but we do have strong indications of that."
Gen Milley also told reporters: "We have very strong evidence that he had a medical history that indicates an unstable psychiatric or psychological condition… we believe that to be a fundamental, underlying causal factor."
He also confirmed that the pistol used in the attack was bought from the same store, Guns Galore in nearby Killeen, where the soldier behind the 2009 gun rampage at Fort Hood purchased his.
Wednesday's shooting took place at two buildings on the base at about 16:00 local time (21:00 GMT).
The gunman walked into one of the buildings and opened fire with a .45-calibre semi-automatic pistol.
He then got into a vehicle and continued firing before entering another building and opening fire again.
It was here that Lopez was eventually stopped by military police, shooting himself in the head when confronted by a female officer in the car park.
The shooting incident lasted between 15 and 20 minutes.
On Thursday, family members told the Chicago Sun-Times newspaper that one of the dead was Army Sgt Timothy Owens, who had reportedly been shot in the chest at close range.
Nine of those injured are being treated in hospital. Three of them were initially in a critical condition but that has since been changed to fair, while a fourth was being released.
The shooting has shocked America's political and military community.
President Barack Obama said: "To see this unspeakable, senseless violence happen in a place where they're supposed to feel safe - home base - is tragic."
Fort Hood was the scene of a gun rampage in 2009 that left 13 soldiers dead and 32 wounded.
Maj Nidal Hasan was sentenced to death last September for that attack.
It was the deadliest attack on a domestic military installation in US history.
The building where the attack took place has only recently been demolished to make way for a place of remembrance.
Another US soldier, Muslim convert Naser Jason Abdo, was jailed for life for plotting to blow up a restaurant full of troops near Fort Hood in 2011.