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The US Army has said the gunman, who opened fire at Fort Hood military base, was mentally unstable and that was believed to be a "fundamental, underlying cause" of the shooting incident.
Speaking at a news conference, base commander Lt. Gen. Mark Milley said a verbal altercation with another person at the post "may have immediately preceded the shooting". He confirmed the gunman, who yesterday killed his three colleagues, was 34-year-old Ivan Lopez from Puerto Rico.
Milley said it was not confirmed whether Lopez targeted specific people. But, he said, "There is no indication at this time he was targeting specific individuals."
"At this point we have not yet ruled out anything whatsoever," he said, adding that there are no indications that Lopez had any links to terrorist organisations, either national or international.
Milley said Lopez purchased the gun - a .45 calibre Smith and Wesson semi-automatic pistol - on March 1 from Guns Galore, the same gun store in Killeen from where Major Nidal Hasan bought weapons he used in a mass shooting in 2009.
In 2011, an Army private from Kentucky, Naser Jason Abdo, had bought from the same store gun powder he used to make a shrapnel bomb with which he intended to kill soldiers in Killeen. Lopez had not registered a weapon, Milley said.
Extending condolences to the families of the victims, Milley said an investigation is ongoing. He said that of the 16 people injured, some remain hospitalised at Darnell Army Medical Center and at Scott and White Hospital - Temple, which took in the most critically injured.
Four patients were discharged from Scott and White Hospital in Temple, while five others remain, said Beth Valvano, a hospital spokeswoman. Three who were in critical condition have been upgraded to serious, and two other patients are in good condition, she said.
A memorial ceremony is planned for next week, but Milley said a time and date would be announced later.
