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.
Lieutenant General Mark Milley formally identified him as Specialist Ivan Lopez, 34, who later shot himself. Sixteen others were wounded.
He told reporters there was "very strong evidence" that Lopez, originally from Puerto Rico, had an unstable psychological condition.
The BBC reports Lopez served in Iraq and was being assessed for post-traumatic stress disorder.
There may have been a verbal altercation with another soldier or soldiers," General Milley said on Thursday at Fort Hood.
"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."
General Milley also said: "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 Major Nidal Hasan purchased his weapon for a rampage at Fort Hood in 2009.
Thirteen soldiers were killed and 32 wounded. Hasan was sentenced to death last September.
Hasan said he opened fire to protect Taliban insurgents from US troops who were about to deploy to Afghanistan.
The BBC reports Wedbnesday's shooting took place at two buildings on the base at about 4pm local time.
Lopez 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.