Fort Hood commander Lt Gen. Mark Milley told a news conference that the shooter was a serving soldier and that he had died on the scene from self-inflicted gunshot wounds after being confronted by a military police officer.
Milley said there was no indication the incident was linked to "terrorism".
He told reporters the soldier, who he did not name because his family had not yet been informed, had served in Iraq for four months in 2011. The soldier was being treated for depression and anxiety, and was being evaluated for possible post-traumatic stress disorder.
Milley did not draw a link between the gunman's medical evaluation and the shooting.
The AP reported that the soldier had shot himself in the head after being confronted by military police in a car park at the base, after entering two buildings and firing.
As he came within six metres of an officer, he put his hands up but then reached under his jacket and pulled out his gun. The officer drew her own weapon but the suspect put the gun to his head and pulled the trigger, the agency reported.
NBC correspondent Jay Gray told Al Jazeera that it was believed a.45 calibre Smith and Wesson pistol used in the shooting had been recently purchased from the local area and was not a military weapon.
Investigators, including the Federal Bureau of Investigation, army investigators, state and local police, were examining whether the incident was related to a disagreement that had escalated between two soldiers, Gray said.
The suspect's wife was cooperating with investigators, an FBI official said, according to CNN.
In an inititial response to the attack, US President Barack Obama promised a full investigation.